Detection in Analytical Chemistry - ACS Publications - American

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Chapter 16

Real-World Limitations to Detection 1

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David A. Kurtz , John K. Taylor , Larry Sturdivan , Warren B. Crummett , Charles R. Midkiff, Jr. , Robert L. Watters, Jr. , Laura J. Wood , W. William Hanneman , and William Horwitz 5

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Department of Pathology, Pennsylvania State University, University Park, PA 16802 National Bureau of Standards, Gaithersburg, MD 20899 CBM Branch, Chemical Research and Development Center, U.S. Army Chemical Research, Aberdeen Proving Ground, MD 21010 Analytical Laboratories, Dow Chemical Company, Midland, MI 48667 National Laboratory Center, Bureau of Alcohol, Tobacco, and Firearms, Rockville, MD 20850 Center for Analytical Chemistry, National Bureau of Standards, Gaithersburg, MD 20899 Center for Technology, Kaiser Aluminum and Chemical Corporation, 6177 Sunol Boulevard, Pleasanton, CA 94566 Food and Drug Administration, Washington, DC 20204 2

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Data obtained at or near the limit of detection is often improperly prepared by scientists and improperl y interpreted by the receiving public. This panel offers advice to scientists preparing data to have s t a t i s t i c a l support for the data produced. Four examples from the real world illustrate interpretation problems: limit of detection reporting, the r o l e of matrix effects and matrix v a r i a b i l i t y i n forensic work, the effects of the blank in ICP work, and federal regulatory problems. Transferring c a r e f u l l y prepared data from the laboratory to the general public can result i n problems and d i f f i c u l t i e s often not considered beforehand. Data are created by analysts, usually at the request of others. These data are of varying quality but are used i n many areas of l i f e . Though the general purpose i s to solve problems, problems are often created because of the varying q u a l i t i e s of the data and the varying backgrounds of the users of the data. We have convened a panel to discuss some of these problems suggesting i n some cases solutions that could a l l e v i a t e them as problems. There are four groups of people who use a n a l y t i c a l information in daily l i f e . These are the non-experts (everyone except the analysts), the governmental regulators and those regulated, the rulers of the courts, and the analysts themselves. Each of these has 0097-6156/88/0361-0288$08.25/0 © 1988 American Chemical Society

In Detection in Analytical Chemistry; Currie, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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d i f f e r e n t backgrounds, d i f f e r e n t areas of t h o u g h t , and d i f f e r e n t purposes i n u s i n g a n a l y t i c a l i n f o r m a t i o n . The i n t e r a c t i o n between t h e s e g r o u p s on s u c h i s s u e s c a n l e a d t o d i s c u s s i o n , f e a r , and t h r e a t s . T h i s panel w i l l show how t h i s can come about. The w r i t e r s each address s p e c i f i c areas where t h e r e have been problems when d i v e r s e people a r e u s i n g a n a l y t i c a l i n f o r m a t i o n . Dr. T a y l o r a d d r e s s e s the a n a l y s t s because t h i s i s of the f i r s t importance. Good i n f o r m a t i o n must come f i r s t . Dr. S t u r d i v a n p i c k s up t h i s theme i n f u r t h e r d i s c u s s i n g how to o b t a i n good i n f o r m a t i o n i n the narrower f i e l d of c a l i b r a t i n g i n s t r u m e n t a l response t o amounts or c o n c e n t r a t i o n s i n samples. When a n a l y t i c a l r e p o r t s a r e made p u b l i c , m i s u n d e r s t a n d i n g s may o c c u r amongst the " n o n - e x p e r t s " as o u t l i n e d by Dr. Crummett. He d i s c u s s e s problems i n such a r e a s as the presence of d i o x i n i n human f a t and c o n t a m i n a t i o n of water s u p p l i e s w i t h t o x i c solvents. Dr. Crummett d i s c u s s e s the r i s k s of f a l s e n e g a t i v e s and f a l s e p o s i t i v e s . The i n t e r p r e t a t i o n and c a l c u l a t i o n of the l i m i t of d e t e c t i o n when t a k i n g the r i s k s i n t o account has not been r e a d i l y a v a i l a b l e . A c c o r d i n g l y , a s h o r t s e c t i o n i s i n c l u d e d h e r e , perhaps i n s i m p l i f i e d form, t o a c q u a i n t the reader w i t h the term, l i m i t of detection. F u r t h e r r e a l - w o r l d problems i n the f o r e n s i c f i e l d are d i s c u s s e d by Mr. M i d k i f f . He d e s c r i b e s some examples a t the l e g a l i n t e r f a c e . In t h i s a r e a the a n a l y s e s a r e even more d i f f i c u l t because o f the u n u s u a l m a t r i c e s found and the l a c k of sample t o work w i t h . The governments, of c o u r s e , are asked by the people t o become a c t i v e i n t h i s a r e a because of t h e i r concerns and l a c k of knowledge of b o t h the a r e a s of a n a l y s i s and t o x i c o l o g y . Dr. Hanneman has shown t h a t b r o a d l y w r i t t e n r e g u l a t i o n s can be d i f f i c u l t t o comply w i t h . Here i s a l s o an example where r e s e a r c h e r s have not read the F e d e r a l R e g i s t e r , the c e n t r a l r e g u l a t i o n document, c a r e f u l l y enough t o get i t s t r u e meaning. Hence, they can get the wrong i m p r e s s i o n and s u p p o r t an e r r o n e o u s p l a n of a c t i o n . F i n a l l y , Dr. H o r w i t z takes the o t h e r s i d e of the c o i n and defends the government i n p u t t i n g out good documentat i o n about t h e i r w e l l planned r e g u l a t i o n s . He a l s o recommends some b a s i c p o i n t s f o r a n a l y s t s t o remember as r u l e s o f t h e thumb t o f o l l o w . . . a s l o n g as t h e y f i r s t know and f o l l o w good a n a l y t i c a l practice. The f o l l o w i n g t e x t d i s c u s s e s problem a r e a s . Here, a n a l y s t s are w o r k i n g i n t r a c e amounts o r a t the l i m i t of d e t e c t i o n . Frequently, problems o c c u r due t o the i n t e r a c t i o n between the a n a l y s t s , c o u r t s , and n o n - e x p e r t s . Key q u e s t i o n s t h a t might be answered a r e : *** Can a q u a l i t y l a b e l be put on data? *** How can we t e a c h the p u b l i c the concept o f error? *** Can we d i s c u s s d a t a , t a k i n g e r r o r i n t o a c c o u n t , w i t h o u t i n d u c i n g an a l a r m i s t r e a c t i o n ? *** How do we u n d e r s t a n d the s p e c i a l d i f f i c u l t i e s of o b t a i n i n g a n a l y s i s d a t a r e l a t i n g t o f o r e n s i c problems· *** Can we i n t e r a c t w i t h f e d e r a l r e g u l a t o r s i n the making of s u i t a b l e r e g u l a t i o n s ? B a s i c Data Q u a l i t y ( D r . J . K. Have we

always had

Taylor)

i n the l a b o r a t o r y a s t a t e of s t a t i s t i c a l

In Detection in Analytical Chemistry; Currie, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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on a g i v e n problem? Dr. T a y l o r says i n t h i s r e g a r d , " S t a t i s t i c a l c o n t r o l of the measurement p r o c e s s i s the f i r s t requirement f o r meani n g f u l d e t e c t i o n , s i n c e w i t h o u t i t a l l d a t a are m e a n i n g l e s s . This means t h a t a l i m i t i n g mean must be r e a l i z e d and a s t a b l e s t a n d a r d d e v i a t i o n must be a t t a i n e d and m a i n t a i n e d . " By l i m i t i n g mean we have i n mind a mean t h a t approaches the t r u e mean as the sample s i z e becomes i n f i n i t e . A s t a b l e s t a n d a r d d e v i a t i o n means one t h a t does not change i n s i z e w i t h the a d d i t i o n of a d d i t i o n a l samples. In o t h e r words an a n a l y s t must be a b l e to reproduce h i s r e s u l t s a t a l l t i m e s . O t h e r w i s e , any r e s u l t a t any time i s m e a n i n g l e s s . " S t a t i s t i c a l c o n t r o l a p p l i e s to a l l p a r t s of the a n a l y t i c a l s y s t e m - s a m p l i n g p r o c e s s , t h e c a l i b r a t i o n , t h e b l a n k , and the measurement. S t a t i s t i c a l c o n t r o l i s a t t a i n e d by the q u a l i t y c o n t r o l of the e n t i r e system and i n v o l v e s maintenance of r e a l i s t i c t o l e r a n c e s for a l l c r i t i c a l operations. A system of c o n t r o l c h a r t s i s the b e s t way to demonstrate a t t a i n m e n t of s t a t i s t i c a l c o n t r o l and to e v a l u a t e the a p p r o p r i a t e standard d e v i a t i o n s . In the s i m p l e s t form, the r e s u l t s of measurement of a s t a b l e check sample, o b t a i n e d over a p e r i o d of t i m e , are p l o t t e d . S t a t i s t i c a l c o n t r o l i s demonstrated when the v a l u e s are randomly d i s t r i b u t e d around t h e i r average v a l u e . " C o n t r o l l i m i t s are o f t e n taken as + 2 or 3 s t a n d a r d d e v i a t i o n u n i t s of these r e p l i c a t e s . Dr. T a y l o r a l s o adds, "Even the ranges of d u p l i c a t e measurements of the a c t u a l samples t e s t e d can be p l o t t e d i n a s i m i l a r manner to demonstrate a s t a b l e s t a n d a r d d e v i a t i o n . In e i t h e r c a s e , t h e s t a t i s t i c s o f the c o n t r o l c h a r t s a r e t h e b e s t d e s c r i p t o r s of the v a r i a b i l i t y of the measurement p r o c e s s . " D r . T a y l o r c o n t i n u e s h i s g e n e r a l a d v i c e : " L a t e r speakers w i l l d i s c u s s how measurement v a r i a b i l i t y q u a n t i t a t i v e l y d e f i n e s the l i m i t s of d e t e c t i o n and q u a n t i f i c a t i o n . Due to the n a t u r e of measurement, each l a b o r a t o r y ( a n a l y s t ) w i l l have somewhat d i f f e r e n t measurement u n c e r t a i n t i e s , and hence d i f f e r e n t l i m i t s of d e t e c t i o n . Published v a l u e s o f LOD's ( L i m i t s Of D e t e c t i o n ) , MDL's ( M e t h o d D e t e c t i o n L i m i t s ) , or what-have-you f o r methodology are t y p i c a l , a t b e s t ; hence they have no p r e d i c t a b l e q u a n t i t a t i v e r e l a t i o n to those o b t a i n e d by any l a b o r a t o r y or a n a l y s t . Each must e v a l u a t e them f o r i t s e l f and w i l l make somewhat d i f f e r e n t d e c i s i o n s c o n c e r n i n g p r e c i s i o n and d e t e c t i o n when a n a l y z i n g the same samples. Each has the p r o f e s s i o n a l o b l i g a t i o n to o b t a i n a l l i n f o r m a t i o n n e c e s s a r y to support the q u a l i t y of i t s d a t a , which must be t e c h n i c a l l y sound and d e f e n s i b l e . "Measurement u n c e r t a i n t y becomes c r i t i c a l l y i m p o r t a n t as i t i n f l u e n c e s the d e c i s i o n p r o c e s s . In h i g h - a c c u r a c y a n a l y s i s , t h i s i n f l u e n c e s the l a s t s i g n i f i c a n t f i g u r e . In t r a c e a n a l y s i s , i t i n f l u e n c e s the f i r s t ( w h i c h i s a l s o the l a s t ) s i g n i f i c a n t f i g u r e . No f i g u r e can be s i g n i f i c a n t w i t h o u t s t a t i s t i c a l s u p p o r t . Luckily, many measurements are made w i t h more s i g n i f i c a n t f i g u r e s than are needed f o r a d e c i s i o n p r o c e s s . A n a l y s t s must remember t h a t they cannot use s t a t i s t i c s o b t a i n e d by any one e l s e t o support t h e i r own d a t a and hence i n making d e c i s i o n s u s i n g i t . "A f a c t t h a t i s n o t a l w a y s a p p r e c i a t e d i s t h a t t h e use o f a p p r o p r i a t e methodology i s n e c e s s a r y but not s u f f i c i e n t f o r r e l i a b l e measurement d a t a and/or f o r a t t a i n i n g s p e c i f i e d l i m i t s of d e t e c t i o n . In the EPA Love Canal s t u d i e s , f o r example, the c o n t r a c t l a b o r a t o r i e s v a r i e d by more than a f a c t o r of ten i n t h e i r d e t e c t i o n l i m i t s w h i l e u t i l i z i n g the same methodology. A c c o r d i n g l y , i t i s f a l l a c i o u s to

In Detection in Analytical Chemistry; Currie, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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a s s i g n q u a l i t y l a b e l s to d a t a based on t y p i c a l or even c o l l a b o r a t i v e test r e s u l t s . Data from v a r i o u s l a b o r a t o r i e s may be of b e t t e r or p o o r e r q u a l i t y than such i n d i c a t o r s . Only r e l i a b l e e s t i m a t e s based on the performance of the producer should be u s e d . "The wide d i f f e r e n c e s t h a t can o c c u r i n the q u a l i t y of d a t a have serious i m p l i c a t i o n s for data compilations. U n l e s s t h e r e i s some way to code and/or to a s s o c i a t e d a t a w i t h i t s u n c e r t a i n t y , poor d a t a can be unduly i n f l u e n t i a l i n subsequent d a t a a n a l y s i s . In d e t e c t i o n s i t u a t i o n s i m p r e c i s e d a t a l e a d s to l a r g e d e t e c t i o n l i m i t s ; hence non-detection can have a d i f f e r e n t meaning, depending on how the measurement was made. " I n t h i s s p e a k e r s o p i n i o n the p r e c i s i o n a t t a i n e d i n measuring r e a l samples [ed.: as opposed to measurement of s t a n d a r d s o n l y or s t a n d a r d s d i s s o l v e d i n s u b s t i t u t e background m a t r i c e s ] i s the o n l y r e l i a b l e b a s i s f o r d e c i s i o n s on d e t e c t i o n . I n l a r g e measurement programs, the use of d u p l i c a t e - s a m p l e c o n t r o l c h a r t s i s the most f e a s i b l e way to e s t a b l i s h the p r e c i s i o n parameters needed and to defend l i m i t s of d e t e c t i o n . O t h e r w i s e , a s u f f i c i e n t number o f r e p l i c a t e measurements must be made on the samples t e s t e d f o r t h i s purpose. Without documented d e m o n s t r a t i o n of p r e c i s i o n , the d a t a are meaningless." 1

C a l i b r a t i o n E r r o r s (Dr. L.

Sturdivan)

Another a s p e c t of d a t a q u a l i t y t h a t needs to be addressed i s the e r r o r t h a t i s produced d u r i n g the c a l i b r a t i o n s t e p i n a n a l y s i s . W h i l e , s t r i c t l y s p e a k i n g , c a l i b r a t i o n e r r o r s are s l i g h t l y o u t s i d e the r e a l m of d e t e c t i o n l i m i t e r r o r s , i t i s a l s o t r u e t h a t much a n a l y t i c a l work i s done a t the d e t e c t i o n l i m i t and the c a l i b r a t i o n e r r o r s t h a t are produced can be p r o p o r t i o n a t e l y much l a r g e r a t the d e t e c t i o n l i m i t than elsewhere on the g r a p h . Hence, i t i s most a p p r o p r i a t e to d i s c u s s the m a t t e r at t h i s t i m e . Most q u a n t i t a t i v e a n a l y t i c a l d a t a r e s u l t s from a c a l i b r a t i o n step. W h e t h e r t h e d a t a comes f r o m a gas c h r o m a t o g r a p h , l i q u i d chromatograph or from a s p e c t r o m e t e r , c a l i b r a t i o n i s r e q u i r e d . C a l i b r a t i o n r e l a t e s i n s t r u m e n t a l response w i t h a s p e c i f i e d amount of substance to be measured. C a l i b r a t i o n r e q u i r e s s t a n d a r d s to cover and enclose the entire concentration range of interest. In p r e p a r a t i o n of these s t a n d a r d s , however, Dr. S t u r d i v a n comments: "Often the s t a n d a r d samples are made by s e r i a l d i l u t i o n . They're t r e a t e d as though they are independent samples," w h i c h they a r e n ' t i n t h a t c a s e . "Now t h e r e i s n o t h i n g wrong w i t h making s e r i a l d i l u t i o n s , but i f t h a t i s done, i t would be d e s i r a b l e to make two s e r i e s of s e r i a l d i l u t i o n s , s i d e by s i d e . " What he i s r e f e r r i n g to comes from the f o l l o w i n g d i s c u s s i o n : The b e s t method f o r the p r e p a r a t i o n of s t a n d a r d s i s to prepare each c o n c e n t r a t i o n d i r e c t l y ( o r w i t h s e r i a l d i l u t i o n s ) from neat (100% pure) compounds ( w i t h the same number of d i l u t i o n s t e p s ) . This can be c o s t l y i n b o t h time and m a t e r i a l s . The next b e s t method, and one t h a t can be shown to be s t a t i s t i c a l l y n e a r l y c o r r e c t i s to s t a r t from two d i f f e r e n t neat s t a n d a r d s and d i l u t e them i n p a r a l l e l t r e e - l i k e s t e p s . Tree s t r u c t u r e d i l u t i o n s would be to p r e p a r e AA and AB from A; then p r e p a r e AAA, AAB, and AAC from AA and ABA, ABB, and ABC from AB, e t c .

In Detection in Analytical Chemistry; Currie, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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A n o t h e r approach i n sample p r e p a r a t i o n , and one h a v i n g some b i a s , i s t o s t a r t from one neat w e i g h i n g measurement a s s u r i n g t h a t each s t a n d a r d has the same number o f d i l u t i o n s t e p s i n i t s p r e p a r a tion. The d i l u t i o n s t e p s would be o f a t r e e s t r u c t u r e . Thus each d i l u t i o n s t a n d a r d w i l l have t h e same p r e p a r a t i o n v a r i a n c e . The most common s t a n d a r d s p r e p a r a t i o n method i s one where each s t a n d a r d i s s e r i a l l y produced from t h e p r o c e e d i n g one. I n t h i s case a l a r g e b i a s i s developed as d i l u t i o n s p r o c e e d . The e r r o r v a r i a n c e of d i l u t i o n g e t s l a r g e r and l a r g e r w i t h each d i l u t i o n s t e p . D r . S t u r d i v a n c o n t i n u e s , "There may be d i l u t i o n e r r o r s . These tend t o o c c u r more f r e q u e n t l y a t the lower end, down t h e r e where t h e d e t e c t i o n l i m i t i s . I f t h e r e i s a d s o r p t i o n t o the w a l l s o f the c o n t a i n e r , the l o w e s t s t a n d a r d s resemble b l a n k s . " Dr. S t u r d i v a n emphasizes "Good c o n f i d e n c e l i m i t s around t h e c a l i b r a t i o n graph must be done i n e q u a l v a r i a n c e space·" What he i s s a y i n g i s the f o l l o w i n g : C a l i b r a t i o n s t a n d a r d s a r e prepared a t v a r i o u s c o n c e n t r a t i o n l e v e l s t o c o v e r the d e s i r e d a n a l y t i c a l r a n g e . The square o f the s t a n d a r d d e v i a t i o n , the v a r i a n c e , a t each l e v e l must be a p p r o x i m a t e l y e q u a l . A q u i c k l o o k a t any chromatographic c a l i b r a t i o n d a t a c o v e r i n g a wide range w i l l show t h a t t h i s c o n d i t i o n i s not met. Some s o r t o f treatment must be done w i t h the d a t a i n o r d e r t o have an e q u a l - v a r i a n c e c o n d i t i o n . Transformation of response data has been found to s a t i s f y the e q u a l v a r i a n c e requirement ( 1 ) He c o n t i n u e s : "Often the c a l i b r a t i o n graphs a r e l i n e a r over a p a r t i c u l a r r e g i o n but not over the r e g i o n o f i n t e r e s t . Therefore, one s h o u l d check the l i n e a r i t i e s and use a n o n - l i n e a r c a l i b r a t i o n graph as w e l l . " I f f i t t i n g to a l i n e a r function i s desired ( f o r s i m p l i c i t y ' s s a k e ) , the t r a n s f o r m a t i o n f u n c t i o n used f o r t h e amount v a r i a b l e s need n o t be the same as t h a t used on the response v a r i a b l e s f o r e q u a l v a r i a n c e . I n t h i s manner d a t a can o f t e n be brought i n t o c o n f o r m i t y w i t h the s i m p l e r l i n e a r f u n c t i o n C O . "Okay, what a r e the b e t t e r s o l u t i o n s ? Use e q u a l v a r i a n c e space. Put c o n f i d e n c e l i m i t s around t h a t graph i n e q u a l v a r i a n c e space. You may have t o do a t the b e g i n n i n g a l i t t l e more c a l i b r a t i o n , make a few more s t a n d a r d s , and put a l i t t l e more work i n t o e s t a b l i s h i n g those c a l i b r a t i o n g r a p h s . Don't t r y t o extend t h e c u r v e beyond your r e g i o n of i n t e r e s t . I t i s n o t r e a s o n a b l e t o a s s o c i a t e the same v a r i a n c e w i t h t h e b l a n k s t h a t you a s s o c i a t e w i t h t h e c u r v e . And t h e r e f o r e , one would have t o do a l i t t l e more work w i t h the b l a n k s and determine what t h e d e c i s i o n l i m i t i s b a s e d o n . I f the d i l u t i o n e r r o r i s s i g n i f i c a n t , determine the ' d e c i s i o n l i m i t ' on the b a s i s o f b l a n k s alone· "What w o u l d t h e i d e a l system c o n s i s t o f ? W e l l , f i r s t o f a l l we would determine what t h e c o s t o f the Type I e r r o r ( f a l s e p o s i t i v e ) i s and what the c o s t o f a Type I I e r r o r ( f a l s e n e g a t i v e ) i s . Then we would determine the p r o b a b i l i t y o f e n c o u n t e r i n g those particular errors. The t h i r d s t e p i s t o determine the d i s t r i b u t i o n o f f u t u r e samples (whether u n i f o r m , n o r m a l , o r w h a t ) . F i n a l l y one can m i n i m i z e the p r o b a b l e c o s t o f f u t u r e e r r o r s . " These l a b o r a t o r y problems as r e l a t e d i n t h i s f i r s t p a r t o f the p a n e l d i s c u s s i o n a r e i m p o r t a n t t o s o l v e i n o r d e r t o o b t a i n good w o r k i n g d a t a . However, when the d a t a g e t s put i n t o the p u b l i c a r e n a ,

In Detection in Analytical Chemistry; Currie, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

16. KURTZ ET AL. look out sections.

Real- World Limitations to Detection

f o r more problems.

A

few

are d e s c r i b e d

293 i n the

next

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Limit of Detection and Public Trust (Dr. W. B. Crummett) "Over the past several years great progress has been made by the s c i e n t i f i c community working largely through s c i e n t i f i c societies to define and understand the meaning of the ' l i m i t of detection' and the ' l i m i t of q u a n t i f i c a t i o n ' . Thus, the American Chemical Society has issued guidelines (2) and principles (3) of environmental a n a l y s i s . The A s s o c i a t i o n of O f f i c i a l A n a l y t i c a l Chemists (AOAC) and the American Society for Testing Materials (ASTM) have continued to emphasize collaborative studies, cooperating with the International Union of Pure and Applied Chemistry (IUPAC) i n holding symposia and studying best ways to conduct such studies," writes Dr. Crummett. He continues: "In spite of this extraordinary e f f o r t , however, analysts i n the r e a l world continue to present their results i n forms which sometimes cause the c r e d i b i l i t y of the data to be questioned or the meaning to be misinterpreted. The problem becomes much more serious as the ' l i m i t of detection' i s approached." Either the c r e d i b i l i t y of science i s put into question or the wrong impression i s given to the public i n one manner or another. The biggest problem with interpretation by the public i s the general public lack of understanding about uncertainty. If I have three apples i n my basket, then the person on the street knows there are three apples there because he can count them. However, i f a measurement of 2 ppm for an analysis of pesticide "x" i n my drinking water i s reported, he expects that not only was there exactly 2 ppm in the sample but that there i s exactly and always 2 ppm there, now and anytime l a t e r . Our schools simply do not teach uncertainty and change, but they should...way down i n the lower grades. The f i r s t example Dr. Crummett talks about refers to analyses of dioxin compounds i n human f a t . Ten years ago analyses were done at the ppm l e v e l , and- there was uncertainty i n those figures, both i n i d e n t i f i c a t i o n and i n q u a n t i f i c a t i o n . More recently, levels were being reported i n the high ppb range. At this time quantities of dioxin i n the medium and low ppt (parts per t r i l l i o n , picograms/g) ranges are being reported. These are 1000 to 1,000,000 times more sensitive than the best analyses previously done. Here i s the f i r s t example : "Look at the results i n a study sponsored by the Environmental Protection Agency, EPA, and conducted by P h i l Albro of the National Institute of Environmental Health Sciences, NIEHS ( 4 ) . Eight i n t e r nationally known laboratories participated i n the study, each using their favorite method. Human fat was spiked with various concentrations of three PCDD's and three PCDF's at low parts per t r i l l i o n l e v e l s . As part of the data set, the number of unusually low and unusually high values were reported. These were the number of values that deviated by 50% from the spiked amount i n the sample." A t o t a l of 54 samples were reported i n this example from each laboratory. The data are found i n Table I. The v a r i a b i l i t y of analysis i n this case i s quite pronounced, but i t may be understandable i n view of the extremely low l e v e l of

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294

Table I . EPA/NIEHS C o l l a b o r a t i v e Study o f D i o x i n i n Human F a t . Number o f V a l u e s Above and Below 50% o f Spiked Amount i n 54 A n a l y s e s

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No. of Low V a l u e s No. i n Mid Range No. of High Values

1 0 50 4

A n a l y t i c a l Method, L a b o r a t o r i e s 7 6 4 5 2 3 7 8 10 1 0 0 43 31 42 47 54 52 16 2 6 3 2 0

8 0 51 3

a n a l y t e p r e s e n t and the number o f a n a l y s i s s t e p s t h e p r o c e s s r e quired. Dr. Crummett c o n c l u d e s i n t h i s c a s e : " A l t h o u g h such p e r f o r mance i s s u f f i c i e n t f o r some s c i e n t i f i c s t u d i e s i n which t r e n d s a r e the g o a l , i t i s not q u a n t i t a t i v e enough f o r d e c i s i o n - m a k i n g when regulation i s indicated." Data o f t h i s s o r t , however, can be handled i n r e g u l a t o r y cases i f the proper s t a t i s t i c a l c a l c u l a t i o n f o r the d e c i s i o n l i m i t and t h e l i m i t o f d e t e c t i o n i s performed (_5). I n r e g u l a t o r y cases a t t e n t i o n i s g i v e n t o the s t a n d a r d d e v i a t i o n s o f t h e b l a n k and t h e a n a l y t e d e t e c t e d i n t h e r e g i o n o f the l i m i t . Similar risks for false posit i v e s and f a l s e n e g a t i v e s ( a and 3) s h o u l d be s p e c i f i e d . As i t t u r n s o u t , however, i n the example above c i t e d , t h e r e were p r o b a b l y l a b o r a t o r y procedures a f f e c t i n g t h e v a r i a b i l i t y t h a t s h o u l d have been examined b e f o r e t h e d a t a were s u b m i t t e d f o r r e g u l a t o r y purposes. A n a l y s i s problems f o r d i o x i n a t the p a r t s p e r t r i l l i o n l e v e l i n human f a t would be s i m i l a r t o p a r t s per q u a d r i l l i o n (1000 times more remote) i n water because w i t h water one works u s u a l l y w i t h a much l a r g e r sample s i z e . S i n c e few work a t the p a r t s per q u a d r i l l i o n l e v e l i n w a t e r , i t i s expected t h a t the d i o x i n problem s h o u l d be extremely severe. Another s t u d y quoted by Dr. Crummett i n v o l v e d a n a l y s i s a t t h i s l e v e l i n f i v e p r i v a t e water w e l l s performed by f o u r d i f f e r e n t l a b o r a t o r i e s . As Dr. Crummett r e l a t e s : "Some o f the consequences and c o n f u s i o n o f u s i n g such methodology can be seen from a s t u d y the EPA attempted i n 1984 t o determine i f 2,3,7,8-TCDD had contaminated p o t a b l e water i n t h e M i d l a n d , MI a r e a . The r e s u l t s o f t h i s s t u d y a r e shown i n Table I I . Table I I . 2,3,7,8-TCDD A n a l y s i s i n F i v e Water W e l l s performed by Four L a b o r a t o r i e s over a P e r i o d o f Time. A n a l y s i s i n pg/L ( l i m i t o f d e t e c t i o n , pg/L) Sampling Date November, 1984 December, 1984 June, 1985 August, 1985 September, 1985

Lab 1 (a) ND(5-50)

Lab 2

ND (10) ND

(2)

Lab 3

ND (8) ND (1) ND (1)

Lab 4

ND (10) ( a ) ND ( 1 0 ) ND ( 3 )

(a) L a b o r a t o r y sample c o n t a m i n a t i o n p r e s e n t

In Detection in Analytical Chemistry; Currie, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

16.

KURTZ ET AL.

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295

"The f i r s t l a b o r a t o r y was r e p o r t e d to have contaminated the f i r s t s e t of samples, and i t o b t a i n e d out of c o n t r o l d e t e c t i o n on the second s e t . A t h i r d sampling was sent to t h r e e d i f f e r e n t l a b o r a ­ t o r i e s , u s i n g s u p e r i o r methodology. S t i l l , some c o n t a m i n a t i o n and v a r i a b i l i t y was e x p e r i e n c e d , and the w e l l s had t o be sampled t w i c e more. The f o l l o w i n g f r o n t page h e a d l i n e i n the M i d l a n d D a i l y News o f f e r e d one r e s o l u t i o n t o e r u d i t e p e o p l e who may have s e e n t h e v a r i a b l e l i m i t s of d e t e c t i o n f o r these a n a l y s e s : 'EPA Says those who Don't T r u s t W e l l Water Should Buy i t i n B o t t l e s . (6) " I t c a n be shown t h a t p e o p l e c a n have a g r e a t c o n c e r n when v a r i a b l e sampling and a n a l y t i c a l work i s known t o them. However, t h i s example a l s o shows a l a c k of u n d e r s t a n d i n g on the p a r t of the media and the p u b l i c c o n c e r n i n g the u n c e r t a i n t y and d i f f i c u l t y of such an a n a l y s i s . " Dr. Crummett concludes h i s d i s c u s s i o n , "The use of i n a p p r o p r i a t e sampling a n a l y t i c a l methodology or f a u l t y t e c h n i q u e b r i n g s f a l s e numbers i n t o the d a t a s e t of the l a b o r a t o r y r e p o r t . These may then be communicated to the p u b l i c . M i s i n t e r p r e t a t i o n s and f a u l t y r e a d i n g of the l a b o r a t o r y r e p o r t may a l s o cause h e a d l i n e s i n newspapers which convey a message c o n t r a r y to the d a t a i n the i n i t i a l l a b o r a t o r y report. " A l l f a l s e n e g a t i v e r e s u l t s thus generated l e a d t o a f a l s e sense of s e c u r i t y . F a l s e p o s i t i v e r e s u l t s , on the o t h e r hand, l e a d t o an e x p e n s i v e c o u r s e of i n v e s t i g a t i o n which i s sure to i n v o l v e one or more governmental a g e n c i e s and produce unwarranted f e a r i n the gener­ a l p u b l i c . Are we a n a l y t i c a l chemists t o t a l l y r e s p o n s i b l e f o r t h i s unnecessary a c t i v i t y and f e a r ? We are p r o b a b l y n o t , a l t h o u g h we are o f t e n accused of i t . "

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1

L i m i t of D e t e c t i o n R e v i s i t e d ( D r . D. A.

Kurtz)

At t h i s p o i n t , perhaps some e f f o r t should be made i n s i m p l i f i e d terms to d e s c r i b e a method of c a l c u l a t i n g a l i m i t of d e t e c t i o n i n s t a t i s ­ t i c a l terms f o r t y p i c a l l y generated c a l i b r a t i o n d a t a . The b a s i c problem of t h i s l i m i t , LOD, i s t h a t i t i s not b e i n g d e a l t w i t h i n the o v e r a l l measurement. An LOD i s a s i m p l e number c o n t a i n i n g s t a t e d r i s k s so t h e r e i s no danger i n r e p o r t i n g i t to the p u b l i c . Its α and 3 have t o be shown, and t h i s c o u l d be done i n terms of i t s r i s k . Too f r e q u e n t l y the d e t e c t i o n d e c i s i o n l e v e l ( " c r i t i c a l l e v e l " ) and the d e t e c t i o n l i m i t are c a l c u l a t e d to be one and the same w i t h the r e s u l t t h a t the f a l s e n e g a t i v e ends up h a v i n g a r i s k of 50%, infra. Dr. C u r r i e has d e s c r i b e d t h i s i n simple terms (5): "Our b a s i c t a s k i s to d i s t i n g u i s h the b l a n k or background from a t r u e s i g n a l at the d e t e c t i o n l i m i t . [That] can be done, p r o v i d e d t h a t the s i g n a l s are random, independent, and s t a t i o n a r y . To c o m p l e t e l y s p e c i f y the f a l s e p o s i t i v e (a) and f a l s e n e g a t i v e (3) r i s k s , we must know the form of the [ s i g n a l ] d i s t r i b u t i o n and i t s p a r a m e t e r s . For most a n a l y t i c a l s i t u a t i o n s we assume the d i s t r i b u t i o n to be normal ( G a u s s i a n ) , and the d i s p e r s i o n parameter i s s i m p l y the i m p r e c i s i o n (standard d e v i a t i o n ) . " F o l l o w i n g t h i s argument we must f i r s t a c c e p t the l e v e l s of r i s k , α and 3 · C o n v e n t i o n a l l y , we choose e q u a l r i s k s a t the 5% l e v e l . Hence, α = 3= 5%. I f we s e l e c t Student's t - s t a t i s t i c s i n p l a c e of

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296

DETECTION IN ANALYTICAL CHEMISTRY

z - s t a t i s t i c s because of l i m i t e d sample s i z e , we d e f i n e the d e c i s i o n l e v e l ( L ) and d e t e c t i o n l i m i t ( L ~ ) as shown i n the f o l l o w i n g s k e t c h : r

0

L

c

L

D

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The e q u a t i o n s of the two l i m i t s are as L

=

C

E s t i m a t e d Net S i g n a l follows:

(la)

σ

Ί-α ο

(lb)

The s t a n d a r d d e v i a t i o n o f the b l a n k , σ , i s r e l a t e d t o the s t a n d a r d d e v i a t i o n o f the net sample e s t i m a t e (στ) near the l i m i t o f d e t e c t i o n by the r e l a t i o n : 0

σ

Λ

-

a /

1 + 1/n

σ

β

where η i s the number of r e p l i c a t e s used t o e s t i m a t e the b l a n k ( B ) . S i n c e the r i s k s α and b are chosen t o be of e q u a l v a l u e , the c a l c u l a ­ t i o n f o r L i s s i m p l y 2 L f o r the case when the s c a l e r e f e r s t o the net ( b l a n k - c o r r e c t e d ) s i g n a l ( i . e . , has a mean of z e r o ) . Observed values t h a t are l e s s than L a r e s i m p l y r e p o r t e d as b e i n g n o t d e t e c t e d because the f a l s e p o s i t i v e r i s k has been exceeded from the chosen amount. True v a l u e s l e s s than L a r e not d e t e c t a b l e because the f a l s e n e g a t i v e r i s k exceeds 3· p

L i m i t a t i o n s on the I n t e r p r e t a t i o n of F o r e n s i c L a b o r a t o r y R e s u l t s (Mr. C. R. M i d k i f f ) The v a l u e of the l i m i t of d e t e c t i o n i n f o r e n s i c work i s shrouded i n d e t a i l s not u s u a l l y p r e s e n t i n o t h e r t r a c e a n a l y s i s e f f o r t s . I n t h i s a r e a the performance o f the a n a l y s t r e l a t e s t o samples s u b m i t t e d as l e g a l e v i d e n c e , perhaps f o r some c r i m e . The sample may o f t e n be i n such s h o r t s u p p l y t h a t o n l y l i m i t e d a n a l y s e s can be performed on i t . In these cases the l i m i t of d e t e c t i o n can be g r e a t l y a f f e c t e d . Mr. M i d k i f f i n t r o d u c e s t h i s t o p i c i n the f o l l o w i n g manner: "Recently, i n c r e a s i n g concerns have been addressed about the r e l i a b i l i t y of r e s u l t s from f o r e n s i c l a b o r a t o r i e s . One q u e s t i o n of p a r t i c u l a r concern which may be asked i s the e x t e n t t o which these poor r e s u l t s are a t t r i b u t a b l e to attempts t o push d e t e c t i o n l i m i t s beyond the l i m i t s of ' r e a s o n a b l e s c i e n t i f i c c e r t a i n t y ' , making i n t e r ­ p r e t a t i o n of r e s u l t s tenuous. "The f o r e n s i c chemist i s concerned not o n l y w i t h conduct of an a n a l y s i s and p r e p a r a t i o n of a r e p o r t , but a l s o w i t h the p r e s e n t a t i o n of the r e s u l t s i n a c o u r t of law. C o n c l u s i o n s and o p i n i o n s of the a n a l y s t may have economic impact on i n d i v i d u a l s and o r g a n i z a t i o n s r a n g i n g f r o m d e n i a l o f i n i t i a l employment t o l o s s o f a c u r r e n t

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p o s i t i o n or the n e c e s s i t y f o r payment of s i z e a b l e monetary damages. For i n d i v i d u a l s accused of a c r i m e , the p o t e n t i a l e f f e c t s a r e even more s e r i o u s , i n v o l v i n g l o s s of l i b e r t y o r l i f e i t s e l f . " I n the courtroom such a n a l y t i c a l q u e s t i o n s as the f o l l o w i n g can be asked of the a n a l y t i c a l chemist d u r i n g c r o s s - e x a m i n a t i o n p r o c e d u r e s : "Were the v a l u e s which you c l a i m t o have o b t a i n e d c l o s e to the d e t e c t i o n l i m i t f o r the method used? What i s the d e t e c t i o n l i m i t f o r ( m a t e r i a l ) i n (sample t y p e ) u s i n g (method)? Is that d e t e c t i o n l i m i t based upon e x a m i n a t i o n of a pure sample o r one l i k e the one w h i c h you examined i n t h i s case? Are you f a m i l i a r w i t h the IUPAC method (7) f o r the d e t e r m i n a t i o n of the d e t e c t i o n l i m i t ? " The Sample i n F o r e n s i c Work. " I n the c o n t e x t of c o n v e n t i o n a l a n a l y t i c a l c h e m i s t r y , " Mr. M i d k i f f r e l a t e s , "a method i s designed t o o p t i mize the d e t e r m i n a t i o n of a p a r t i c u l a r a n a l y t e i n a d e f i n e d m a t r i x . A l t h o u g h the c o m p o s i t i o n of the m a t r i x may v a r y , the v a r i a t i o n i s n o r m a l l y w i t h i n an expected range, and the o v e r a l l c o m p o s i t i o n of the m a t r i x i s r e p r o d u c i b l e . As a r e s u l t , the e f f e c t s of the m a t r i x on the d e t e r m i n a t i o n are c o n s i d e r e d i n the d e s i g n o f the a n a l y t i c a l method. Once a d e t e r m i n a t i o n of d e t e c t i o n l i m i t i s made, the a n a l y s t may be c o n f i d e n t t h a t i f the m a t e r i a l of i n t e r e s t i s p r e s e n t i n the sample above a m i n i m a l l e v e l , i t w i l l be d e t e c t e d and measured w i t h an a c c e p t a b l e l e v e l of p r e c i s i o n and a c c u r a c y . "In an a n a l y t i c a l s i t u a t i o n t y p i c a l of a q u a l i t y o r p r o c e s s c o n t r o l l a b o r a t o r y , adequate sample i s p r o v i d e d f o r a l l t e s t s t o be conducted More sample may be o b t a i n e d i f needed. I n a d d i t i o n , the sample m a t r i x i s u s u a l l y known and i t s e f f e c t s u n d e r s t o o d . "By c o n t r a s t , i n the f o r e n s i c l a b o r a t o r y , the q u a n t i t y of sample i s o f t e n l i m i t e d and a d d i t i o n a l m a t e r i a l i s u n o b t a i n a b l e . As a r e s u l t , t h e r e i s l i t t l e o p p o r t u n i t y f o r a n a l y t i c a l methodology modif i c a t i o n and m a t r i x adjustment or o p t i m i z a t i o n . F u r t h e r , the m a t r i x i t s e l f i s v a r i a b l e as i n cases i n v o l v i n g a r s o n . " A d d i t i o n a l l y , c u r r e n t l e g a l t r e n d s r e q u i r e t h a t not a l l the e v i d e n c e be consumed i n the t e s t i n g p r o c e s s . The c o u r t s have, i n r e c e n t y e a r s , taken a dim v i e w of s i t u a t i o n s i n which the e n t i r e sample of e v i d e n t i a r y m a t e r i a l was consumed d u r i n g a n a l y s i s and u n a v a i l a b l e f o r e x a m i n a t i o n by opposing e x p e r t s . " Matrix Effects. S i g n a l S u p p r e s s i o n and Enhancement. Major e f f e c t s on the LOD can be found due t o the u n a v o i d a b l e presence of the sample m a t r i x . These e f f e c t s can take the form of s u p p r e s s i o n , enhancement, or masking o f the a n a l y t e s i g n a l . S u p p r e s s i o n and enhancement w i l l be d i s c u s s e d here and masking i n the next s e c t i o n . Without t a k i n g these e f f e c t s i n t o a c c o u n t , major e r r o r s can be made i n the c o n c l u s i o n s drawn from the e v i d e n c e . Mr. M i d k i f f w r i t e s : " S i g n a l s u p p r e s s i o n r e f e r s t o e i t h e r a dec r e a s e i n the o v e r a l l a n a l y t i c a l s i g n a l o r s u p p r e s s i o n of the s i g n a l at s e l e c t i v e a n a l y t i c a l w a v e l e n g t h s . M a t r i x - r e l a t e d s i g n a l suppress i o n i s a common problem i n m i n e r a l a n a l y s i s by atomic s p e c t r o s c o p y . In a s t u d y u s i n g I n d u c t i v e l y Coupled Plasma, ICP, atomic e m i s s i o n , i t was found t h a t w h i l e most a n a l y t e s e n s i t i v i t i e s were depressed by m a t r i x e f f e c t s — some up t o 30%, l i t h i u m c o u l d be e i t h e r suppressed o r enhanced depending on s m a l l changes i n c o n d i t i o n s ( 8 ) . In a s i m i l a r s t u d y of low-power ICP, a l l m e t a l s s t u d i e d were suppressed by

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i n c r e a s i n g sodium c o n c e n t r a t i o n except a r s e n i c w h i c h was enhanced (9). " S e l f - a b s o r p t i o n and s a t u r a t i o n e f f e c t s a l s o l e a d t o s u p p r e s s i o n of t h e a n a l y t i c a l s i g n a l . I n gamma o r X-ray a n a l y s i s , s e l f - a d s o r p t i o n i s a f u n c t i o n o f sample t h i c k n e s s . S i g n i f i c a n t s e l f - a b s o r p t i o n i n samples a n a l y z e d by X-ray e m i s s i o n c o u l d r e s u l t i n f a i l u r e t o d e t e c t elements p r e s e n t as t r a c e s o r i n d i c a t e t h e i r c o n c e n t r a t i o n t o be below t h e a c t u a l l e v e l . "For n o n - d e s t r u c t i v e gamma measurements, a mock-up o f t h e o b j e c t b e i n g examined i s o f t e n used f o r c a l i b r a t i o n purposes. Standards o f the elements measured a r e i n c o r p o r a t e d i n t o t h e mock-up which simul a t e s t h e geometry and c h e m i c a l c o m p o s i t i o n o f t h e samples as c l o s e l y as p o s s i b l e . " A l t h o u g h t h e a n a l y s t may be unaware o f i t s e x i s t e n c e i n t h e sample, s i g n a l s u p p r e s s i o n s e r v e s t o decrease system s e n s i t i v i t y and r a i s e s t h e working d e t e c t i o n l i m i t . S p e c t r a l enhancement, on t h e o t h e r hand, i n c r e a s e s t h e system s e n s i t i v i t y and l o w e r s t h e working detection l i m i t . "Enhancement i s a w e l l known problem i n atomic absorption spectroscopy (10). A v a r i e t y o f approaches, such as Zeeman e f f e c t c o r r e c t i o n have been proposed f o r i t s e l i m i n a t i o n ( 1 1 ) . To a v o i d a r t i f i c i a l l y h i g h r e s u l t s , c a l i b r a t i o n s t a n d a r d s must c o n t a i n c o n c e n t r a t i o n s o f t h e enhancing s p e c i e s e q u i v a l e n t t o those i n t h e sample. O r d i n a r y s t a n d a r d s o l u t i o n s a r e n o t r e p r e s e n t a t i v e o f the a n a l y t i c a l s i t u a t i o n . " S p e c t r a l o v e r l a p i s a s p e c i a l case o f enhancement. " S p e c t r a l o v e r l a p can cause d e t e c t i o n and measurement o f a m a t e r i a l n o t p r e s e n t i n the sample (10) and g i v e f a l s e p o s i t i v e * r e s u l t s . Detection l i m i t s must be r e c o g n i z e d as b e i n g i n s e p a r a b l y l i n k e d t o s e l e c t i v i t y because r e l i a b l e d e t e c t i o n l i m i t s cannot be e s t a b l i s h e d u n l e s s t h e r e i s c e r t a i n t y i n what i s b e i n g measured." T

M a t r i x E f f e c t s . S i g n a l Masking. S i g n a l masking r a i s e s t h e e f f e c t i v e d e t e c t i o n l i m i t and s e r i o u s l y c o m p l i c a t e s i n t e r p r e t a t i o n o f t h e a n a l y t i c a l data. One cause o f s i g n a l masking i s e l e v a t i o n o f t h e background s i g n a l l e a d i n g t o u n f a v o r a b l e s i g n a l / n o i s e r a t i o s . "An example i s t h e e x a m i n a t i o n o f swabs c o l l e c t e d from t h e hands o f a suspected s h o o t e r . Antimony and barium a r e r e l a t i v e l y uncommon i n nature or manufactured products but are present i n the primer c o m p o s i t i o n o f most t y p e s o f modern a m m u n i t i o n . D u r i n g weapon d i s c h a r g e o r h a n d l i n g , these elements a r e d e p o s i t e d on t h e hands and can s u b s e q u e n t l y be c o l l e c t e d w i t h a c i d - m o i s t e n e d c o t t o n swabs. U n f o r t u n a t e l y , o t h e r m a t e r i a l s p r e s e n t on t h e hands, such a s , d i r t , g r e a s e , o i l o r b l o o d a r e a l s o c o l l e c t e d by t h e swabbing p r o c e s s . " I n t h e l a b o r a t o r y , t h e swabs a r e l e a c h e d w i t h n i t r i c a c i d t o e x t r a c t t h e barium and antimony f o r FAAS a n a l y s i s . L i g h t swabbing r e s u l t s i n i n e f f e c t i v e e x t r a c t i o n o f these elements from t h e swabs. The a n a l y s t w i l l o b t a i n a ' f a l s e n e g a t i v e ' r e s u l t f o r t h e presence o f f i r e a r m s d i s c h a r g e r e s i d u e s . I f l o n g e r l e a c h i n g times o r a g i t a t i o n of t h e swab a r e used, contaminants cause a background e l e v a t i o n t o an e x t e n t n o t r e a d i l y noted o r c o r r e c t a b l e , even w i t h s o p h i s t i c a t e d instrumentation. "Masking c a n a l s o o c c u r when b l i n d l y r e l y i n g upon s o p h i s t i c a t e d instruments. F o r example, many modern atomic a b s o r p t i o n i n s t r u m e n t s p r o v i d e f o r d i s p l a y o f o n l y one s i g n a l a t a t i m e , e i t h e r t h e back-

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ground alone o r the background c o r r e c t e d s i g n a l . The a n a l y s t r e l y i n g on automatic background c o r r e c t i o n may be unaware o f the magnitude o f the background s i g n a l b e i n g s u b t r a c t e d (12,13) · Background s u b t r a c t i o n , r e g a r d l e s s o f system used, does n o t change t h e s i g n a l / n o i s e r a t i o i n the system. I t cannot, and s h o u l d n o t , be r e l i e d upon t o c o n v e r t a sample u n f i t f o r a n a l y s i s i n t o one from which important i n f o r m a t i o n and c o n c l u s i o n s w i l l be drawn." Matrix V a r i a b i l i t y . L i m i t s of d e t e c t i o n of v a r i o u s analytes are dependent n o t o n l y on t h e m a t r i x but a l s o on t h e changeable q u a l i t i e s of the m a t r i x due t o the i n c i d e n t . T h i s l a t t e r q u a l i t y i s seen i n a d d i t i o n o f a n a l y t e s from p y r o l y s i s and the s u b t r a c t i o n o f a n a l y t e s from a d s o r p t i o n onto p y r o l y z e d m a t e r i a l . Mr. M i d k i f f c o n t i n u e s , "When a sample from a suspected a r s o n i s examined by gas chromatography, a d d i t i o n a l peaks from m a t e r i a l s p r e s e n t a t t h e scene, as f o r example, p l a s t i c s , i n t h e sample may be o b s e r v e d . These a d d i t i o n a l peaks make d i f f i c u l t p a t t e r n r e c o g n i t i o n , normally relied upon f o r d e t e c t i o n / i d e n t i f i c a t i o n o f flammable l i q u i d s i n the d e b r i s . S i m i l a r problems may be encountered i n the a n a l y s i s o f samples from a bomb scene where c h e m i c a l s i n s o i l o r d e b r i s from the bomb c r a t e r c o m p l i c a t e t h e d e t e c t i o n and i d e n t i f i c a t i o n o f e x p l o s i v e components. "When t h e m a t e r i a l s o u g h t i s n o t d e t e c t e d i n an a d s o r b e n t c o l l e c t e d sample, as i n the case o f a c h a r c o a l sample produced by f i r e , p a r t i a l o r t o t a l r e t e n t i o n by the a d s o r b e n t may be a f a c t o r . C o n v e r s e l y , t h e adsorbent may have v a r y i n g e f f e c t i v e n e s s i n r e t a i n i n g t h e a n a l y t e because o f s u r f a c e i n a c t i v a t i o n , water s a t u r a t i o n , o r some o t h e r cause (14) ." Conclusions. Mr. M i d k i f f concludes w i t h these remarks. " I n t h e f o r e n s i c l a b o r a t o r y , t o ensure a r e a s o n a b l e i n t e r p r e t a t i o n o f t h e a n a l y t i c a l d a t a , t h e a n a l y s t must have c e r t a i n t y i n what i s b e i n g measured. There must be a f o c u s on a n a l y t i c a l f u n d a m e n t a l s , and t h e a n a l y s t must be c o g n i z a n t o f f a c t o r s i m p a c t i n g d e t e c t i o n and measurement. These f a c t o r s i n c l u d e m a t r i x e f f e c t s i n t h e a c t u a l sample; p o t e n t i a l c o n t a m i n a t i o n o r l o s s d u r i n g sample h a n d l i n g , s t o r a g e , and a n a l y s i s ; and i n s t r u m e n t s e n s i t i v i t y . The a n a l y s t must be s u f f i c i e n t l y f a m i l i a r w i t h the m a t e r i a l t o determine t h e p r o b a t i v e v a l u e of s i m i l a r i t i e s and t h e d i s c r i m i n a t i o n v a l u e o f minor d i f f e r e n c e s . The a n a l y s t s h o u l d r e c o g n i z e t h a t p r e s e t c r i t e r i a may negate the need f o r f i n e r d a t a and a v o i d a tendency t o attempt t o use lower and lower values. The next d e c i m a l p l a c e may be uncharted ground and j u s t because i t c a n be measured, does n o t g i v e i t p r o b a t i v e v a l u e . While improved d e t e c t i o n l i m i t s o f f e r advantages i n t h e e x a m i n a t i o n of p h y s i c a l e v i d e n c e , the a d d i t i o n a l d a t a r e q u i r e s c a r e i n i n t e r p r e t a tion. The r e s u l t s may have a major impact a f f e c t i n g an i n d i v i d u a l ' s f o r t u n e , freedom, o r l i f e . To ensure f a i r n e s s and overcome q u e s t i o n s about t h e i r c r e d i b i l i t y , f o r e n s i c s c i e n t i s t s must c o n s i d e r t h e problems t h a t a r e c r e a t e d by improved d e t e c t i o n l i m i t s and t h e i n t e r p r e t a t i o n of r e s u l t s i n the f o r e n s i c l a b o r a t o r y . "

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The Role o f the Blank i n t h e Measurement o f D e t e c t i o n L i m i t s i n Atomic Spectrometry ( D r . R. L. W a t t e r s , J r . and L. J . Wood) Some o f t h e problems i n f o r e n s i c a n a l y s i s r e l a t e d t o sample m a t r i x and d e t e c t i o n l i m i t s can be found i n o t h e r a n a l y t i c a l l a b o r a t o r i e s as well. B e s i d e s t h e enhancement and s u p p r e s s i o n e f f e c t s on a n a l y t e s i g n a l s d e s c r i b e d by Mr. M i d k i f f i n the p r e v i o u s s e c t i o n , Dr. Watters and Ms. Wood o f t h e N a t i o n a l B u r e a u o f S t a n d a r d s p r o v i d e some s p e c i f i c i n f o r m a t i o n r e g a r d i n g m a t r i x e f f e c t s on s p e c t r a l background. Background l e v e l and s p e c t r a l s t r u c t u r e comprise t h e c h i e f source o f measured s i g n a l when b l a n k s a r e b e i n g measured f o r d e t e c t i o n l i m i t estimation. T h e i r example i s taken from i n d u c t i v e l y c o u p l e d plasma (ICP) spectrometry. Dr. Watters w r i t e s : "A d e t e c t i o n l i m i t (DL) i s u s u a l l y used f o r comparison, and the s p e c i f i c type o f comparison o f t e n d i c t a t e s how the DL i s measured o r c a l c u l a t e d . When v a l i d b l a n k measurements and c a l c u l a t i o n s a r e made, such as those d e s c r i b e d by C u r r i e ( 1 5 ) , t h e r e s u l t i n g DL i s a p p l i c a b l e o n l y t o t h a t s p e c i f i c a n a l y t i c a l system and c o n d i t i o n s used f o r the DL d e t e r m i n a t i o n . Erroneous c o n c l u s i o n s f r e q u e n t l y a r i s e when t h i s same DL i s used as a comparison f o r o t h e r systems o r c o n d i t i o n s . " One way t o ensure t h a t DL's a r e used o n l y f o r v a l i d comparisons i s t o examine the s p e c i f i c r e a s o n f o r a p a r t i c u l a r DL e s t i m a t i o n . Watters and Wood c o n t i n u e : "To examine t h e v a l i d i t y o f DL c o m p a r i sons, we can pose two q u e s t i o n s t h a t a r e supposed t o be answered by a DL e s t i m a t e : ( 1 ) I s one i n s t r u m e n t b e t t e r than another f o r d e t e r m i n i n g a t r a c e a n a l y t e ? ( 2 ) Can an a n a l y t e be determined i n a s p e c i f i c sample m a t r i x u s i n g a s p e c i f i c method? "The u s u a l approach t o answering e i t h e r o f these q u e s t i o n s i s based on e s t a b l i s h i n g t h e i n s t r u m e n t response w i t h r e s p e c t t o a n a l y t e c o n c e n t r a t i o n and measuring t h e v a r i a b i l i t y o f t h i s response when no analyte i s present. Some a n a l y s t s p r e f e r t o c a l l t h e m e a s u r e d response t h e ' b l a n k . The s o l u t i o n used f o r the measurement i s a l s o c a l l e d the 'blank' , and i t i s the c o m p o s i t i o n o f t h i s b l a n k t h a t o f t e n l e a d s t o t h e problems mentioned above. For s i m p l i c i t y of d i s c u s s i o n , we w i l l c o n s i d e r samples and b l a n k s t o be s o l u t i o n s . " A l t h o u g h the d i s t i n c t i o n i s seldom made, t h e r e a r e a t l e a s t t h r e e d i f f e r e n t types o f ' z e r o - a n a l y t e ' samples t h a t a r e c a l l e d 'blanks'. The f i r s t i s t h e s o l u t i o n t h a t c o n t a i n s o n l y r e a g e n t s mixed i n q u a n t i t i e s t h a t a r e used i n the c a l i b r a t i o n s t a n d a r d s o r t h e f i n a l sample d i l u t i o n . T h i s ' s o l v e n t ' o r 'reagent' b l a n k i s sometimes used as the 'zero s t a n d a r d ' t o d e f i n e t h e y - i n t e r c e p t o f t h e c a l i b r a t i o n curve. "When samples a r e prepared u s i n g d i s s o l u t i o n methods, t h e t r u e a n a l y t i c a l b l a n k c o n s i s t s o f a l l r e a g e n t s and s t e p s used i n t h e method. The o n l y a n a l y t e p r e s e n t i n t h i s second type o f b l a n k i s caused by c o n t a m i n a t i o n from any reagent o r c o n t a c t w i t h l a b o r a t o r y environment and a p p a r a t u s . The l e v e l o f a n a l y t e i n t h i s ' a n a l y t i c a l ' b l a n k and i t s v a r i a b i l i t y a r e key q u a n t i t i e s t o be e v a l u a t e d i n a c c u r a t e t r a c e a n a l y s i s ( 1 6 ) . The c o n t e n t o f the a n a l y t i c a l b l a n k i s more method dependent than t h a t o f the reagent b l a n k . "The t h i r d type o f s o l u t i o n b l a n k i s one t h a t c o n t a i n s e v e r y component o f a s p e c i f i c s a m p l e e x c e p t t h e a n a l y t e o f i n t e r e s t . R a r e l y does one encounter such a s o l u t i o n i n t h e normal c o u r s e o f 1

In Detection in Analytical Chemistry; Currie, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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analysis, and indeed i t i s d i f f i c u l t to make such a solution by design. The usefulness of having this type of 'matrix* blank w i l l be discussed i n terms of detection l i m i t measurements using inductively coupled plasma (ICP) spectrometry as an example." The example chosen to demonstrate the s i g n i f i c a n t differences i n detection l i m i t estimates obtained by measuring d i f f e r e n t types of blank solutions i s taken from ICP spectrometry. However, analogous situations arise i n any form of spectrometry where net absorption or emission peaks are measured. Dr. Waiters describes the relationship between the questions posed above and the type of blank to be mea­ sured for DL estimation: "If one wishes to answer question 1 above, replicate ICP mea­ surements are made with or without spectral background correction, while aspirating a solution of d i l u t e pure acid i n d i s t i l l e d water. The choice of whether or not to use background correction may be dictated by the ICP spectrometer vintage, i . e . some older instruments are not capable of background measurement on either or both sides of the analyte spectral peak. Accurate measurements near the detection l i m i t require background correction. Detection l i m i t s should there­ fore be estimated with net intensity measurements of the blank. In t h i s case, the q u e s t i o n of instrument comparison i s adequately answered by measuring the reagent blank, provided a l l measurement parameters are r e a l i s t i c and similar from instrument to instrument. On the other hand, i f one has to answer question 2, DL estimates using the reagent blank can y i e l d erroneously low DL's and use of an a n a l y t i c a l or matrix blank i s more appropriate. The reason more complex solutions result i n higher detection l i m i t s i s that certain components of variance are proportional to the l e v e l of signal being measured. Any increase i n background l e v e l , spectral l i n e caused by contamination, or spectral l i n e feature caused by an i n t e r f e r i n g matrix element l i n e can r e s u l t i n an i n c r e a s e i n the d e t e c t i o n limit." This effect may be offset i f the increase i n blank measure­ ment variance i s compensated by matrix suppression of the c a l i b r a t i o n curve as previously mentioned by Mr. M i d k i f f . Ms. Wood describes how certain types of DL estimates were made u s i n g ICP spectrometry and d i f f e r e n t kinds of blank s o l u t i o n s : "Regardless of the mathematical expression one uses to estimate a DL, the term common to many approaches i s the standard deviation of the background, σ . This term i s estimated by r e p l i c a t e measurements of the background as s . We have examined the e f f e c t s of blank type on the value of s, f o r a number of spectral analysis lines using the ICP. Where differences occur from one blank type to another, the underlying reasons f o r the differences are examined. Five d i f f e r e n t blanks were prepared as examples of reagent, a n a l y t i c a l , and matrix blanks. The reagent blank consisted of 1% V/V HÎTCL i n d i s t i l l e d water. An a n a l y t i c a l blank was prepared using a d i s s o l u t i o n method for s t e e l samples. Another a n a l y t i c a l blank was prepared using a procedure suitable f o r dissolving geological samples by lithium metaborate fusion. Matrix blanks were prepared using 99.999% pure iron i n one and 99.999% pure aluminum i n the other. In each case, 1 g of metal was dissolved i n 100 mL of 10% V/V 12M HC1. A sequential ICP spectrometer was used to measure net signals for the elements and spectral l i n e s l i s t e d i n Table I I I . Ten measurements of each of the

In Detection in Analytical Chemistry; Currie, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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f i v e s o l u t i o n s were made and the s t a n d a r d d e v i a t i o n s of these r e p l i c a t e measurements a t each wavelength are a l s o l i s t e d i n the t a b l e . In cases where two wavelengths are l i s t e d f o r a g i v e n element, the s e c o n d w a v e l e n g t h has b e e n i d e n t i f i e d as one w i t h no s p e c t r a l i n t e r f e r e n c e from i r o n ( 1 7 ) . "The r e a g e n t b l a n k can be used as an i n d i c a t i o n of the b a s e l i n e instrumental detection limit, when i t s s t a n d a r d deviation is m u l t i p l i e d by the a p p r o p r i a t e constant. Data from t h i s s o l u t i o n c o u l d be used to answer q u e s t i o n 1 above, but i n a number of cases cannot be used to answer the second q u e s t i o n . The a n a l y t i c a l b l a n k s can be used as i n d i c a t i o n s of method d e t e c t i o n l i m i t s , w h e r e i n wet ash d i g e s t i o n or a l k a l i f u s i o n procedures are employed to d i s s o l v e the sample. In the case of the f u s i o n b l a n k , t h i s a n a l y t i c a l b l a n k a l s o s e r v e s as a s p e c i f i c type of m a t r i x b l a n k s i n c e the p r i n c i p a l c o n s t i t u e n t s i n s o l u t i o n are l i t h i u m and boron. The c o n c e n t r a t e d i r o n and aluminum s o l u t i o n s r e p r e s e n t s o l u t i o n m a t r i c e s t h a t c o n t a i n the major c o n s t i t u e n t when the d e t e r m i n a t i o n of t r a c e elements i n these m a t r i c e s i s of c o n c e r n . These s o l u t i o n s can be u s e d f o r answering q u e s t i o n 2 above. I n a d d i t i o n , these b l a n k s a l s o s e r v e as a n a l y t i c a l b l a n k s s i n c e d i s s o l u t i o n procedures were employed i n t h e i r making. In some cases the d i f f e r e n c e s i n c a l c u l a t e d d e t e c t i o n l i m i t s among these s o l u t i o n s i s s i g n i f i c a n t . " Dr. Watters p o i n t s out s p e c i f i c examples from the d a t a : " S e v e r a l differences i n standard deviation f o r r e p l i c a t e net intensity measurements between the reagent b l a n k s o l u t i o n and the o t h e r b l a n k s o l u t i o n s can be found i n Table I I I . In g e n e r a l , one can a s c r i b e an i n c r e a s e i n the s t a n d a r d d e v i a t i o n f o r a n a l y t i c a l and m a t r i x b l a n k s compared to the reagent b l a n k to three p o s s i b l e c a u s e s : These are c o n t a m i n a t i o n from the d i s s o l u t i o n p r o c e d u r e , broadband s h i f t s i n the s p e c t r a l background caused by a m a t r i x element, and s p e c t r a l l i n e i n t e r f e r e n c e from a m a t r i x element. Table I I I c o n t a i n s examples of a l l t h r e e of these and t h e i r o c c u r r e n c e i s i n d i c a t e d i n the T a b l e . S p e c i f i c examples can be understood by i n s p e c t i o n of wavelength scans i n the r e g i o n of the b l a n k measurement. "The d i f f e r e n c e s i n background s t a n d a r d d e v i a t i o n f o r the Ca measurements a t 393.366 nm p r o v i d e examples of c o n t a m i n a t i o n and spectral line interference effects. Wavelength scans f o r the f i v e s o l u t i o n s are p l o t t e d i n F i g u r e 1. The e f f e c t of Ca c o n t a m i n a t i o n i n the l i t h i u m metaborate f u s i o n f l u x i s most e v i d e n t . E f f e c t s of t r a c e amounts of Ca c o n t a m i n a t i o n i n d i s s o l v i n g the aluminum and i n the s t e e l b l a n k s o l u t i o n s are a l s o v i s i b l e i n the f i g u r e . The wavelength scan of the i r o n s o l u t i o n c l e a r l y shows the s p e c t r a l i n t e r f e r e n c e from the 393.3605 nm l i n e of Fe. A l t h o u g h the most s e r i o u s degradat i o n i n the Ca d e t e c t i o n l i m i t o c c u r s f o r the f u s i o n b l a n k , t h i s case i s not l i k e l y to cause p r a c t i c a l a n a l y s i s d i f f i c u l t i e s . Fusion t e c h n i q u e s are most o f t e n employed to d i s s o l v e g e o l o g i c a l samples i n which Ca i s u s u a l l y a major c o n s t i t u e n t . Hence t r a c e d e t e c t i o n of Ca i s not a c o n c e r n . "The d e t e c t i o n of Cd at 214.438 nm i s s u b j e c t to i n t e r f e r e n c e s from Fe and A l . I n F i g u r e 2, the s p e c t r a l l i n e i n t e r f e r e n c e of the 214.445 nm l i n e of Fe i s a p p a r e n t , as w e l l as the l a r g e s l o p i n g s h i f t caused by the presence of the h i g h c o n c e n t r a t i o n of A l . T h i s l a t t e r e f f e c t i s even more pronounced f o r the d e t e c t i o n of Cr a t 205.552 nm. The scan p l o t t e d i n F i g u r e 3 shows the l a r g e continuum s h i f t caused

In Detection in Analytical Chemistry; Currie, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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L i BO

\ - ^ ^ Y 1* HN0

-~ \Fe

A l 10000 ppm Steel

/

3

\

/

10000 ppm 393.366 nm

F i g u r e 1. S p e c t r a l scan of ICP e m i s s i o n i n t e n s i t y v e r s u s wavel e n g t h i n the r e g i o n of the 393.366 nm l i n e o f Ca f o r v a r i o u s types of b l a n k s o l u t i o n s .

F i g u r e 2. S p e c t r a l scans showing the e f f e c t of 10000 ppm Fe and A l on the background near the 214.438 nm l i n e o f Cd.

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F i g u r e 3. The e f f e c t o f 10000 ppm A l and Fe on t h e s p e c t r a l background o f t h e 205.552 nm l i n e o f C r .

In Detection in Analytical Chemistry; Currie, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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T a b l e I I I . S t a n d a r d d e v i a t i o n o f 10 r e p l i c a t e measurements o f t h r e e types o f b l a n k s o l u t i o n s f o r a n a l y t e element d e t e c t i o n u s i n g ICP s p e c t r o m e t r y . Standard d e v i a t i o n s a r e e x p r e s s e d i n u n i t s o f ng/mL

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Element Ca Cd Cr Cr Cu Mg Mg Mo Mo Ni Pb Si Si V Zn a

b

c

d

λ I(nm) 393 214 205 427 327 279 280 202 281 231 220 251 288 311 213

.366 .438 .552 .480 .396 .553 .270 .030 .615 .604 .353 .611 .158 .071 .856

1% HN0 0.,1 9.,6 21 22 4.,5 0..2 1..5 21 14 8..0 81 7,.5 13 3,.2 6 .2

3

Fusion Blank a

1. 0 9.,4 35 18 3.,3 0.,2 3..0 28 7.,0 5..1 70 13 36 2..1 5,.5

Steel Blank 0.1 6.5 19 23 2.3 0.1 0.9 21 8.5 8.6 57 24 18 3.2 5.1 a

Al

Fe b

0.3 38 40 18 86 0.9 1.8 82 8.9 21 148 274 36 13 36 b

b

b

b

b

b

b

b

b

b

a

0.6 20 148 20 4.0 1.0 3.3 76 322 13.9 250 26 13 3.7 8.2 c

c

d

c

c

C o n t a m i n a t i o n from r e a g e n t s o r p r o c e d u r e s Matrix spectral line interference M a t r i x continuum background s h i f t Unidentified spectral line interference

by A l . T h i s phenomenon has been e x p l a i n e d by L a r s o n and F a s s e l ( 1 8 ) , who d e s c r i b e d t h e background s h i f t from r e c o m b i n a t i o n continuum i n the spectrum o f A l . The r e c o g n i t i o n o f t h e i m p o r t a n c e o f t h i s continuum s h i f t l e d t o t h e g e n e r a l acceptance o f t h e need f o r back­ ground c o r r e c t i o n near t h e ICP a n a l y t e e m i s s i o n l i n e . Consequently, ICP s p e c t r o m e t e r s have t h e a b i l i t y t o measure background a d j a c e n t t o a s p e c t r a l l i n e t o c o r r e c t f o r s y s t e m a t i c e r r o r s of q u a n t i f i c a t i o n due t o background s h i f t s . I t i s e v i d e n t here t h a t t h e same phenome­ non a l s o a f f e c t s the random e r r o r component r e l a t e d t o d e t e c t i o n . " I t i s i n t e r e s t i n g t o note t h a t t h e secondary l i n e s f o r C r , Mg, Mo, and S i , chosen t o a v o i d i r o n i n t e r f e r e n c e , do a l l e v i a t e o r reduce the e f f e c t s o f Fe l i n e o v e r l a p . F o r Cr and Mo, s t a n d a r d d e v i a t i o n o f the background f o r the secondary l i n e s i s lower f o r the Fe m a t r i x b l a n k than f o r t h e d i l u t e a c i d b l a n k . T h i s i s p r o b a b l y caused by matrix suppression of the c a l i b r a t i o n curve as d e s c r i b e d by Mr. Midkiff." Dr. Watters c o n c l u d e s : " I t i s e v i d e n t t h a t t h e term ' d e t e c t i o n limit can have m u l t i p l e meanings. The type o f b l a n k t h a t i s used f o r an e x p e r i m e n t a l e s t i m a t e o f t h e d e t e c t i o n l i m i t i s r e l a t e d t o the s p e c i f i c type o f d e t e c t i o n l i m i t d e s i r e d . We have p r e s e n t e d examples taken from ICP s p e c t r o m e t r i c d a t a t h a t i n d i c a t e t h e need f o r c l e a n l a b o r a t o r y sample p r e p a r a t i o n facilities, so t h a t contamination e f f e c t s can be reduced o r e l i m i n a t e d from any type o f d e t e c t i o n l i m i t estimation. Even under t h e b e s t l a b o r a t o r y c o n d i t i o n s , t h e problem 1

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of a n a l y t e d e t e c t i o n i n the presence o f complex m a t r i x elements a t h i g h c o n c e n t r a t i o n s can be e s p e c i a l l y d i f f i c u l t . Detection l i m i t e s t i m a t i o n r e l a t e d t o a s p e c i f i c sample treatment and sample m a t r i x r e q u i r e s the c a r e f u l s e l e c t i o n o f the a p p r o p r i a t e b l a n k t o ensure r e a l i s t i c estimates. So c a l l e d ' i n s t r u m e n t a l d e t e c t i o n l i m i t s ' may be u s e f u l i n comparing one i n s t r u m e n t t o another f o r samples w i t h no interfering species. However, r e a l i s t i c analytical detection q u e s t i o n s must be answered by r e a l i s t i c e s t i m a t i o n procedures and blanks."

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Problems w i t h R e g u l a t o r y

L i m i t S e t t i n g s ( D r . W. W. Hanneman)

[*Note: The quoted o p i n i o n s expressed here a r e those o f the a u t h o r alone] F e d e r a l a d m i n i s t r a t i v e a g e n c i e s r e g u l a r l y have t h e t a s k o f s e t t i n g l i m i t s o f d e t e c t i o n as mandated by l e g i s l a t i v e A c t s t h a t r e q u i r e a n o - d e t e c t i o n l e v e l o f a c h e m i c a l t o be c l a s s e d as " c l e a n " . While a g r e a t d e a l o f o u t s i d e a d v i c e i s sought, r e c e i v e d , and used by the a g e n c i e s , d i f f i c u l t i e s a r i s e when an A c t c o v e r s such a broad range o f a c t i v i t i e s as c h e m i c a l a n a l y s i s . Requirements must cover o r g a n i c s , i n o r g a n i c s , o r g a n o m e t a l l i c s , and b i o c h e m i c a l s ; they must handle g a s e s , s o l u t i o n s , l a r g e m o l e c u l e s and s m a l l . We have a l r e a d y seen some examples i n the s e c t i o n above. However, even i n more common a n a l y t i c a l a r e a s , t h e r e a r e c l a i m s t h a t problems have a r i s e n when an agency has s e t a d e t e c t i o n l e v e l t h a t may be u n a t t a i n a b l e . Method D e t e c t i o n L i m i t . I n EPA, where much o f t h i s work was done, e a r l y attempts a t s e t t i n g d e t e c t i o n l i m i t s were based on the s i g n a l / n o i s e c o n c e p t . I n 1984, came the i n i t i a l e f f o r t s t o u t i l i z e s t a t i s t i c a l c o n c e p t s when the Method D e t e c t i o n L i m i t (MDL) was brought i n t o use. The next s t e p was the Maximum Contaminant L e v e l (MCL). F i n a l l y , the P r a c t i c a l Q u a n t i f i c a t i o n L i m i t (PQL) has been proposed. In at l e a s t some cases these l i m i t s a r e u n a t t a i n a b l e i n the r e a l w o r l d , a c c o r d i n g t o D r . Hanneman, who d e s c r i b e s these problems: "Most t o x i c o l o g i s t s b e l i e v e t h a t the dose makes the p o i s o n . N e v e r t h e l e s s , the EPA and s t a t e r e g u l a t o r y a g e n c i e s , a b e t t e d by a n a l y s t s and the media, have promoted the zero r i s k c o n c e p t . This concept o f f e r s some a t t r a c t i v e advantages. I t reduces the need f o r s c i e n t i f i c judgment, and i t s i m p l i f i e s the r e g u l a t i o n o f c a r c i n o g e n s . The l o g i c a l consequence o f a c c e p t i n g t h i s premise i s t h a t exposure l i m i t s must be s e t t o z e r o . "The zero r i s k concept produced the g r e a t e s t growth a r e a f o r a n a l y s t s i n the h i s t o r y o f mankind. New methods had t o be d e v e l o p e d , l i m i t s o f d e t e c t i o n had t o be lowered, and compliance had t o be demonstrated. A l l t h i s was f u e l e d by the l i n e a r e x t r a p o l a t i o n t o zero. " L e t ' s examine the case o f benzo(a)pyrene ( B a P ) . We c o u l d do the same w i t h PCB's o r almost any suspected c a r c i n o g e n . As promulg a t e d i n 1979, EPA M e t h o d 610 (J_9) i s a p p l i c a b l e t o i n d u s t r i a l discharges. I t was designed t o meet m o n i t o r i n g r e q u i r e m e n t s o f the N a t i o n a l P o l l u t i o n D i s c h a r g e E l i m i n a t i o n System (NPDES) p e r m i t s . I t s l i m i t o f d e t e c t i o n f o r BaP was s t a t e d t o be 40 p a r t s p e r t r i l l i o n u s i n g f l u o r e s c e n c e d e t e c t i o n ( 19) · One o f o u r p l a n t s had a NPDES

In Detection in Analytical Chemistry; Currie, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

16. KURTZ ET AL.

Real- World Limitations to Detection

permit which allowed a discharge of 50 parts per t r i l l i o n of BaP. We had to regularly demonstrate compliance. "The stated Detection Limit of Method 610 was determined from the signal/noise r a t i o measured i n reagent grade water. Since actual detection l i m i t s of methods depend upon the l e v e l of interferences rather than instrumental l i m i t a t i o n s , there i s no compelling reason to expect that we w i l l ever be able to demonstrate compliance u n t i l our plant discharges only reagent grade water. "In 1984 the EPA ' o f f i c i a l l y recognized that one could not make measurements at 'precisely the detection l i m i t so they introduced the concept of MDL (20). 'Definition: The Method Detection Limit (MDL) i s defined as the minimum concentration of a substance that can be measured and reported with a 99% confidence that the analyte concentrations are greater than zero.'" EPA recognized that the MDL w i l l vary depending on instrumental s e n s i t i v i t i e s and matrix e f f e c t s . The reference included a table of representative MDL's determined from reagent water. For BaP the value l i s t e d was 23 ppt (21 ) i n the HPLC method. The reference went on to say that "similar results were obtained using representative waste waters. The MDL actually achieved i n a given analysis w i l l vary depending on instrument s e n s i t i v i t y and matrix effects." Paragraph 15.3 (20) said: "This method was tested by 16 laboratories using reagent water, drinking water, surface water, and three indust r i a l wastewaters spiked at s i x concentrations over a range of 0.1 to 425 yg/L. Single operator precision, o v e r a l l precision, and method accuracy were found to be d i r e c t l y related to concentration of the parameter and e s s e n t i a l l y independent of the sample matrix." Reagent water was defined as water having no background peak i n the v i c i n i t y of the analyte i n question. Quality control acceptance data f o r BaP (21) at a 'detection l i m i t ' test concentration of 10 yg/L gave a standard deviation of 4 yg/L (n=4). This amounts to a Relative Standard Deviation (RSD) of 40%. [Note: One of the biggest problems, however, with the MDL i s that i t ignores errors of the second type, accepting false negatives. The method puts a 50% confidence l e v e l on this type of error since the mean of the d i s t r i b u t i o n of positive values i s three standard deviations (to give 99% confidence) up from zero, the mean of the blank. In order to properly assess values at the 99 % l e v e l i t would be necessary to locate the MDL another 3 standard deviations away from zero. In this position there i s the 99% confidence that when the analyte i s there, i t i s reported and when not there, i t i s not reported.] Dr. Hanneman i l l u s t r a t e s the r e a l data, obtained for setting the MDL f o r BaP, i n Figure 4 (22). "The histograms represent the raw data from Method 610 v a l i d a t i o n , at 200, 2000, and 12,000 parts per trillion. For BaP the RSD ranged from 40 to 53%. Even so, i n order to bring i t down to this l e v e l , the s t a t i s t i c i a n s had to discard about 1/5 of the data. The results at a l l levels were the same. The method d i d not define a l i m i t of detection." And he continues, "This prompts the question, 'What are the performance c r i t e r i a for a v a l i d method?' The answer, i n essence, i s ...a validated method i s one that has undergone the v a l i d a t i o n procedure. There i s no requirement that i t produce meaningful numbers... 1

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and i n the case of b e n z o ( a ) p y r e n e , the method i s e s s e n t i a l l y l e s s f o r measuring a t these l e v e l s . "

worth-

Maximum Contaminant L e v e l . " I n November of l a s t year new measures were proposed. EPA e s t a b l i s h e d the Recommended Maximum Contaminant L e v e l , RMCL (23) , f o r c a r c i n o g e n s and s u s p e c t e d c a r c i n o g e n s . " The RMCL i s the maximum l e v e l of a contaminant i n d r i n k i n g water a t which no known o r a n t i c i p a t e d adverse e f f e c t on the h e a l t h of persons would o c c u r and which i n c l u d e s an adequate margin of s a f e t y . RMCL s a r e non-enforceable. Maximum Contaminant L e v e l s , M C L s , are e n f o r c e a b l e s t a n d a r d s . They a r e s e t as c l o s e as f e a s i b l e t o RMCL's, w i t h the use of the b e s t t e c h n o l o g y , treatment t e c h n i q u e s , and o t h e r means, which the a d m i n i s t r a t o r f i n d s are g e n e r a l l y a v a i l a b l e ( t a k i n g c o s t s i n t o consideration)· D r . Hanneman c o n t i n u e s , "While these MCL's are p r e s e n t l y t o u t e d as d r i n k i n g w a t e r s t a n d a r d s , h i s t o r y t e a c h e s t h a t NPDES p e r m i t w r i t e r s w i l l invoke them. S i n c e our d i s c h a r g e ponds are exposed to the atmosphere, and urban dust i s c o n s t a n t l y c o n t r i b u t i n g BaP to the s u r f a c e , BaP w i l l always be p r e s e n t . We are not p r e s e n t l y i n comp l i a n c e and can never be i n compliance." 1

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T

Practical Quantification Limit. "As of now the LOD i s a dead i s s u e as f a r as suspected c a r c i n o g e n s are concerned. A l l the a n a l y t i c a l t e s t s have been reduced to n o t h i n g more than q u a l i t a t i v e i n d i c a t o r s . What i s even more f r i g h t e n i n g i s t h a t t h e r e i s no way to c o u n t e r a c t a single 'false positive result. "EPA i s now p r o p o s i n g the P r a c t i c a l Q u a n t i f i c a t i o n L i m i t , PQL (24). The PQL i s to be s e t somewhere between 5 and 10 times the MDL." [Note: The PQL was s e t f o r v o l a t i l e o r g a n i c compounds, VOC, i n the f o l l o w i n g manner: Seven of the most e x p e r i e n c e d EPA and c o n t r a c t l a b s r e p o r t e d MDL*s i n the range of 0.2 to 0.5 y g / L . When the same t e s t was repeated i n the same or h i g h e r ranges of c o n c e n t r a t i o n amongst 30-40 o t h e r laboratories, the number of laboratories r e p o r t i n g c o n c e n t r a t i o n s d i f f e r e n t from the t r u e v a l u e was n o t e d . There was a f a i r l y l a r g e number of l a b o r a t o r i e s r e p o r t i n g f i g u r e s d i f f e r e n t from the t r u e v a l u e by more than 20%, but no more than 5 l a b s (6 i n one case) r e p o r t e d a n a l y s i s f i g u r e s d i f f e r e n t from the t r u e v a l u e by more than 40%. Hence, i t was concluded t h a t a h i g h percentage of l a b o r a t o r i e s c o u l d determine these compounds w i t h i n 40% of the t r u e v a l u e . The PQL was determined by t a k i n g the h i g h end of the 7-lab range, 0.5 y g / L , adding 40% of t h a t to o b t a i n 0.7 yg/L and t h e n c h o o s i n g t h e m i d p o i n t o f 5 t o 10 t i m e s t h a t f i g u r e . This amounted to 5 yg/L. Thus, t h i s f i g u r e was determined i n t h i s case w i t h o u t s t a n d a r d d e v i a t i o n s and no RSD can be c a l c u l a t e d . ] " I want to c l o s e by r e i t e r a t i n g what Dr. H o r w i t z demonstrated f i v e y e a r s ago. F i g u r e 5 (25) shows the r e l a t i o n s h i p between the i n t e r l a b o r a t o r y r e l a t i v e s t a n d a r d d e v i a t i o n s , RSD ( o r c o e f f i c i e n t s of v a r i a t i o n ) w i t h c o n c e n t r a t i o n . The i n t e r s e c t i o n of the heavy curve w i t h the 34% RSD l i n e i n d i c a t e s t h a t t h e r e i s no reason to b e l i e v e t h a t one can even d e f i n e a t r u e MDL below approx. 8 ppb, l e t a l o n e at 50 p p t . The i n t e r s e c t i o n w i t h the 20% RSD l i n e i n d i c a t e s t h a t the PQL p r o b a b l y cannot be s e t below 250 ppb. T h i s i s not 5-10 times above the MDL, but r a t h e r 12,000 times t h a t of the 17 ppt v a l u e 1

In Detection in Analytical Chemistry; Currie, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

16. KURTZ ET AL.

TRUE CONC.

12000 PPT

TRUE CONC

J200 PPT

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TRUE CONC.

12,000 PPT

F i g u r e 4. Study.

AVG. RSD =

46%

AVG. BIAS =

-36%

Histograms o f raw d a t a from Method 610 V a l i d a t i o n

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100 PPB F i g u r e 5. H o r w i t z Curve and a n a l y t i c a l

results.

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decreed f o r the n o n - c a r c i n o g e n i c p o l y n u c l e a r a r o m a t i c compound, PNA, benzo(k)fluoranthene. " F i n a l l y , the d a t a p o i n t s shown a r e t y p i c a l of those g e n e r a t e d i n the v a l i d a t i o n s t u d i e s f o r PNA s and PCB's. Mind you, these r e s u l t s were o b t a i n e d a f t e r e l i m i n a t i n g about 20% of the r e s u l t s . " T h e r e i s l i t t l e e v i d e n c e t h a t an RSD o f 20% c a n e v e n be reached. However, the g r e a t e s t p i t y of a l l i s t h a t t h i s a c t i v i t y w i l l i n no way c o n t r i b u t e t o the b e t t e r h e a l t h of the p u b l i c . " 1

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Remarks on D e t e c t i o n L i m i t s ( D r . W.

Horwitz)

When a l l i s s a i d and done about the i n t r i c a t e d e t a i l s of c o r r e c t l y c a l c u l a t i n g l i m i t s of d e t e c t i o n and the l i k e , we need some a d v i c e t o be a b l e to s t a n d back from what we a r e d o i n g j u s t to see the t o t a l effect. T h i s s e c t i o n p r o v i d e s s u c h a d v i c e and i s g i v e n by D r . H o r w i t z , l o n g a mentor i n the f i e l d . One of the most i n t e r e s t i n g and u s e f u l items of a d v i c e i s t o examine r e g u l a r l y the F e d e r a l R e g i s t e r and to respond t o r e q u e s t s f o r i n p u t as a p a r t of the r e g u l a t i o n s e t t i n g p r o c e s s . The quotes below are a l l from Dr. H o r w i t z . F i r s t , A n a l y s i s Purpose. "The f i r s t t h i n g t h a t s h o u l d always be known i n p e r f o r m i n g any a n a l y s i s i s i t s purpose. I n almost a l l cases when d e a l i n g w i t h a l i m i t of d e t e c t i o n o r l i m i t of d e t e r m i n a t i o n , the p r i m a r y purpose of d e t e r m i n i n g t h a t l i m i t i s t o s t a y away from i t . Almost by d e f i n i t i o n , the r e l i a b i l i t y of r e s u l t s near the l i m i t s of d e t e c t i o n or d e t e r m i n a t i o n i s e x t r e m e l y poor and i n f a c t u l t i m a t e l y r e a c h e s a p o i n t where a d e t e r m i n a t i o n o f w h e t h e r an a n a l y t e i s p r e s e n t o r absent can be o b t a i n e d j u s t as r e l i a b l y by t o s s i n g a c o i n as by making the so c a l l e d measurement." Second, I d e n t i f y b e f o r e Measure. "The second i m p o r t a n t p o i n t i s a m a t t e r of d e f i n i t i o n . There are so many p o s s i b l e d e f i n i t i o n s of l i m i t s of d e t e c t i o n t h a t any statement u s i n g these terms s h o u l d always be accompanied by a d e f i n i t i o n as to how the u s e r means i t . ' L i m i t of d e t e c t i o n ' can have t h r e e d i f f e r e n t p r i m a r y meanings whether or not an a n a l y t e (1) was i d e n t i f i e d , (2) measured, o r (3) i d e n t i f i e d and measured. A l t h o u g h measurements can be made w i t h o u t knowing what i s b e i n g measured, they cannot be i n t e r p r e t e d w i t h o u t t h i s i n f o r m a t i o n . The p r o p e r t y of knowing what i s b e i n g measured i s related to the a t t r i b u t e of method s p e c i f i c i t y . A complete a n a l y t i c a l system of s a m p l i n g , d e t e r m i n a t i o n , and interpretation r e q u i r e s t h a t we p l a c e i d e n t i f i c a t i o n b e f o r e measurement. O f t e n i t t a k e s c o n s i d e r a b l y more m a t e r i a l t o i d e n t i f y the presence of an a n a l y t e than t o measure i t s amount. T h e r e f o r e , i n s p e a k i n g of l i m i t s of d e t e c t i o n we are r e a l l y measuring o n l y a naked s i g n a l whose o r i g i n i s obscure. O f t e n we merely assume we know what we are measuring a t these low l e v e l s . This o b v i o u s l y creates considerable ambiguity i n our work." T h i r d , Read the F e d e r a l R e g i s t e r . "There has been c o n s i d e r a b l e c r i t i c i s m w i t h r e g a r d to how r e g u l a t o r y a g e n c i e s u t i l i z e the concept of ' l i m i t s of d e t e c t i o n ' i n v a r i o u s a p p l i c a t i o n s . As background f o r t h i s p o i n t , I would l i k e t o mention t h a t a n a l y t i c a l chemists s h o u l d become f a m i l i a r w i t h t h a t eminent s c i e n t i f i c j o u r n a l c a l l e d the

In Detection in Analytical Chemistry; Currie, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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Federal Register. There i s considerable technical material i n the Federal Register. Its contents include numerous methods of analysis and j u s t i f i c a t i o n f o r these methods. Extensive reviews of the toxicology of various chemical materials appear here, often because the bulk i s too large to publish anywhere e l s e . These reviews have taken many man years for compilation. They are well worth noting. The usual procedure when an agency wishes to promulgate a regulation i s to f i r s t publish a proposal i n the Federal Register accompanied by an i n v i t a t i o n to comment." It should be noted that announcement of these i n v i t a t i o n s i s printed regularly i n Chemical and Engineering News. "Ordinarily 60-90 days are given for receipt of comments, but this can be extended by a formal request. Even so, agencies w i l l c o n s i d e r comments that come i n a f t e r the d e a d l i n e . Regulatory agencies are influenced by these comments. For example, almost 10 years ago the Food and Drug Administration published a proposal known as the ' SOM Document , dealing with trace residues of carcinogenic compounds i n tissues of food-producing animals. Although i t dealt primarily with r i s k analysis, exposure c a l c u l a t i o n s , and carcinogenesis, there was considerable a n a l y t i c a l chemistry involved. As a r e s u l t of e x t e n s i v e comments by the a n a l y t i c a l community, that document was recently republished as a proposal with much of the objectionable a n a l y t i c a l material removed. Consequently, i t i s the obligation of the professional a n a l y t i c a l chemistry community to bring their views to the attention of the responsible regulatory agency to avoid the promulgations of regulations not based on sound science. Sometimes revisions are not possible, because of the way the law i s written. An example i s the case of the Delaney Clause which mandates a zero value. At the time i t was enacted, i t was not realized that attaining zero was impossible. Therefore agencies are attempting to do the best they can to adapt to the situation by developing p r a c t i c a l zeros. 1

"In t h i s connection you should be i n t e r e s t e d i n a recent proposal made by EPA for setting a p r a c t i c a l q u a n t i f i c a t i o n l i m i t for fluoride i n drinking water. They recommend a regulatory l i m i t of 4 ppm of fluoride and a p r a c t i c a l q u a n t i f i c a t i o n l i m i t of 0.5 ppm. For those who are familiar with fluoride determination, the quarrel with this particular value would be that i t i s undoubtedly set too high. You would think that p r a c t i c a l l y a l l laboratories could comfortably determine 0.5 ppm f l u o r i d e . Yet this value was set from actual interlaboratory studies that showed that there were s t i l l a c e r t a i n number of laboratories that could not achieve even such a r e l a t i v e l y l i b e r a l l i m i t of d e t e r m i n a t i o n . I suggest that EPA i s to be congratulated i n coming up with a value for a q u a n t i f i c a t i o n l i m i t that can be p r a c t i c a l l y achieved by most laboratories involved i n this particular determination and recognizing that there i s no need to go any lower (26) · This concept of a p r a c t i c a l q u a n t i f i c a t i o n was f i r s t proposed (24) i n conjunction with v o l a t i l e organic compounds (VOC). It should be examined by a l l who are interested i n the ' l i m i t of detection' concept." Reporting Results. "Another important related matter i s that of reporting low l e v e l r e s u l t s . Again we start with the purpose of the analysis. In some cases an unambiguous answer of yes or no i s what i s desired; i n other cases, a good approximation to the average i s

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needed. The Food and Drug A d m i n i s t r a t i o n ' s t o t a l d i e t program i s a s p e c i f i c example of the need t o know the average i n t a k e of t r a c e n u t r i e n t s and t o x i c contaminants i n the American d i e t . A specific number i s r e q u i r e d f o r i n t e r p r e t a t i o n by t o x i c o l o g i s t s w i t h r e s p e c t to s a f e t y as w e l l as how t h a t v a l u e changes w i t h t i m e . Statements of 'not d e t e c t e d ' or below a c e r t a i n l i m i t of d e t e r m i n a t i o n v a l u e a r e u n s a t i s f a c t o r y because they cannot be handled by d e s c r i p t i v e s t a t i s t i c s i n cases where the amounts p r e s e n t may be below any r e a s o n a b l e d e f i n i t i o n of ' l i m i t of d e t e r m i n a t i o n , ' e s p e c i a l l y when a b l a n k s u b t r a c t i o n i s i n v o l v e d . I n such c a s e s , we can take advantage of the f a c t t h a t i n many i n s t a n c e s the average of a l a r g e number of v e r y poor measurements g i v e s a remarkably good average." T h i s , of c o u r s e , assumes t h a t the e r r o r s are random and not b i a s e d . "That i s i n these cases we a r e d e a l i n g p r i m a r i l y w i t h random e r r o r s of r e l a t i v e l y l a r g e magnitude. In the l o n g run random e r r o r s c a n c e l each o t h e r out l e a v i n g the core of the b a s i c v a l u e we are l o o k i n g f o r . " A l t h o u g h s c i e n t i s t s may have an a v e r s i o n to r e p o r t i n g n e g a t i v e v a l u e s , these are a d i r e c t consequence of the u t i l i z a t i o n of normal d i s t r i b u t i o n s (Figure 6). C o n s i d e r a contaminant p r e s e n t i n a food m a t r i x a t 0.1 ppm w i t h a r e l a t i v e s t a n d a r d d e v i a t i o n (RSD) of 100%. An RSD of 100% means t h a t one s t a n d a r d d e v i a t i o n s u b t r a c t e d from the average g i v e s zero and any v a l u e l e s s than t h i s , say two s t a n d a r d d e v i a t i o n s below the average, i s i n the n e g a t i v e r e g i o n . In f a c t , i n t h i s example 16% of the measurements a u t o m a t i c a l l y w i l l be n e g a t i v e . P h y s i c a l l y t h i s means that the blank measurement was greater than the a n a l y t e measurement. T h i s i s a n a t u r a l consequence of u s i n g the normal d i s t r i b u t i o n . In f a c t i f you do not get about 16% n e g a t i v e v a l u e s i n t h i s s i t u a t i o n , t h e r e i s p r o b a b l y something wrong w i t h your measurements. A sound o b j e c t i o n may be r a i s e d on the grounds t h a t m e a s u r e m e n t s i n t h i s a r e a a r e n o t n o r m a l and t h i s i s a good possibility. However, i n almost a l l cases we never have enough d a t a to determine whether our measurements a r e normal, l o g normal, o r f o l l o w any o t h e r type of d i s t r i b u t i o n . Because of the w e a l t h of i n f o r m a t i o n known about the normal d i s t r i b u t i o n and the many i n s t a n c e s where measurement d a t a have been shown to be n o r m a l l y d i s t r i b u t e d , i t i s g e n e r a l l y best to assume n o r m a l i t y and make any n e c e s s a r y m o d i f i c a t i o n s to the i n t e r p r e t a t i o n i f t h e r e i s evidence to i n d i c a t e non-normality." Summary. "To summarize the major p o i n t s : (1) The u s u a l reason we want t o measure a d e t e c t i o n l i m i t i s t o s t a y away from i t . (2) Report low l e v e l measurements as they come - p o s i t i v e , n e g a t i v e , or zero - s i n c e t h e i r average i s u s u a l l y a r e a s o n a b l y good a p p r o x i m a t i o n of a c t u a l i t y . (3) As we go down i n our c o n c e n t r a t i o n measurements, p r e c i s i o n degenerates i n t o a q u e s t i o n of f a l s e p o s i t i v e s and f a l s e n e g a t i v e s , and a t low enough l e v e l s a t o s s of a c o i n w i l l g i v e us j u s t as good an answer as w i l l our most e l a b o r a t e i n s t r u m e n t a t i o n . (4) At these low l e v e l s , you p r o b a b l y w i l l never have enough d a t a to determine i f you have a normal or non-normal d i s t r i b u t i o n , and when your random e r r o r i s so g r e a t , the type o f d i s t r i b u t i o n p r o b a b l y does not make any d i f f e r e n c e anyway."

In Detection in Analytical Chemistry; Currie, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

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F i g u r e 6. Normal d i s t r i b u t i o n near zero showing the n e c e s s i t y t o report negative values.

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Epilogue R e a l w o r l d problems r e s u l t i n g from a n a l y s i s of s m a l l samples a t the l o w e s t l e v e l s are found i n f o u r a r e n a s : p e r c e p t i o n by the r e a l w o r l d people who are not a n a l y t i c a l e x p e r t s , governmental r e g u l a t i o n s , i n f l u e n c e s of the c o u r t s , and p r o d u c t i o n of the a n a l y t i c a l d a t a by the e x p e r t a n a l y s t . Each of these arenas has i t s own c h a r a c t e r i s t i c s , i t s own o p e r a t i n g r u l e s , and i t s own purposes. We must f i n d , however, enough of a common base so t h a t we can a l l l i v e a m i c a b l y i n t h i s one w o r l d . The person on the s t r e e t , the n o n - e x p e r t , wants a s i m p l e p i e c e of i n f o r m a t i o n . He wants to b e l i e v e what e x p e r t s s a y . Problems i n the p a s t , however, put doubts on what e x p e r t s have s a i d . These doubts a r e c l o u d e d f u r t h e r by a l a c k of knowing about p r o b a b i l i t y r i s k s and r e l e v a n c e . Dr. Crummett has spoken t o t h i s p o i n t . The government r e g u l a t o r s ( l e g i s l a t o r s as w e l l as a d m i n i s t r a t o r s ) l i k e to have neat packages o u t l i n e d by r u l e s t h a t b r o a d l y cover the problem a r e a s . The w o r l d of a n a l y s i s , however, c o v e r s many c o m p l e t e l y d i f f e r e n t a n a l y t i c a l areas - from r a d i o a c t i v i t y c o u n t i n g , to chromatography, and to e m i s s i o n and a d s o r p t i o n a n a l y s i s - t h a t t h i s approach o f t e n becomes i n e f f e c t i v e . Dr. Hanneman's e x p e r i e n c e suggests t h a t r e g u l a t i o n s s h o u l d be problem s p e c i f i c and p h y s i c a l l y t e s t e d under r e a l c o n d i t i o n s b e f o r e p u t t i n g them i n t o use. Rules s h o u l d , p e r h a p s , be b r o a d and u n d e r s t a t e d . They w o u l d be made s p e c i f i c o n l y where unique problems are i d e n t i f i e d t h a t r e q u i r e attachment of s p e c i f i c n u m e r i c a l r e q u i r e m e n t s . The c o u r t s have a v e r y d i f f i c u l t time h a n d l i n g a n a l y t i c a l i n f o r m a t i o n , as Mr. M i d k i f f has e x p l a i n e d . The c o u r t s l o o k a t i n f o r m a t i o n i n a p r e c i s e way, b u t t h e y d o n ' t l i k e e r r o r . E v e r y t h i n g has a s p e c i f i c cause and i t j u s t can't be " p o s s i b l y " t h e r e . A s i d e from e d u c a t i n g a l l the judges and a t t o r n e y s w i t h s t a t i s t i c a l t e c h n i q u e s , the b e s t e f f o r t i s to a s s u r e t h a t the e x p e r t t e s t i m o n y p r o v i d e s v e r y c a r e f u l l y determined i n f o r m a t i o n . Most of the i n f o r m a t i o n p r e s e n t e d i n t h i s p a n e l touches on the r o l e of the a n a l y t i c a l t e c h n i c i a n and r e s e a r c h e r . These i n d i v i d u a l s need to be more s e n s i t i v e to the v a r i o u s amounts of e r r o r t h a t a r e p r e s e n t i n each p a r t of each a n a l y s i s . I t goes a l o n g w i t h the common q u i p i n d a t a h a n d l i n g , "garbage i n , garbage out." I f the a n a l y t i c a l d a t a coming from the l a b o r a t o r y i s p o o r l y c o n t r o l l e d or l a c k s the e s t i m a t i o n s of e r r o r , i t c a n ' t be used by o t h e r s f o r i n f o r m a t i o n l e a d i n g to m e a n i n g f u l d e c i s i o n s and a c t i o n . The f i r s t s u g g e s t i o n , coming from Dr. T a y l o r i s to always put c o n t r o l c h a r t s i n t o each separate t e s t . T h i s t e l l s how the p r o g r e s s of the a n a l y t i c a l t e s t s i s going. Each p e r s o n , each t e s t , and each use of a t e s t needs the control chart. Those u s i n g c a l i b r a t i o n g r a p h s , as Dr. S t u r d i v a n recommends, s h o u l d handle the i n f o r m a t i o n so t h a t t h e r e i s c o n s t a n t v a r i a n c e a t each l e v e l a c r o s s the g r a p h . D r . Crummett has suggested t h a t the a n a l y s t must c l e a r l y s t a t e t h e r i s k s o f m a k i n g a f a l s e d e t e r m i n a t i o n when t r u e and a t r u e d e t e r m i n a t i o n when f a l s e . This i s e s p e c i a l l y important i n s t a t e - o f the-art determinations. Researchers need t o be t e r r i b l y c a r e f u l o f the b l a n k and i t s accompanying u n c e r t a i n t i e s , as p o i n t e d out by Dr. Watters. The r i s k s of making b o t h the f a l s e p o s i t i v e and the f a l s e n e g a t i v e s h o u l d be c l e a r l y d e f i n e d and when so done, s i g n a l s found t o

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be less than the decision l i m i t ( c r i t i c a l l e v e l ) should simply not be reported as present. Blank signals should also be reported. Forensic work, as implied by Mr. Midkiff, requires a great deal of experience because of the wide sample types. The matrix i n such work also has a profound effect on the signal - i t may enhance and i t may mask the signal - and i s variable as well. F i n a l l y , Dr. Horwitz suggests: 1. know the purpose for an analysis before you measure and 2. be aware of what the government has said about the topic. On the other hand, error calculations must be s i m p l i f i e d . There i s a u t i l i t y i n using s i m p l i f i e d , p r a c t i c a l approximations, but i t must be certain that such approximations are clearly noted when used. A restatement of the key questions as key actions i s appro­ priate : *** Put a quality label on data *** Teach the public the concept of error *** Publish data, taking error into account, and l e t the chips f a l l as they may *** Be extremely careful i n producing forensic data so that unequivocal actions can be taken on account of them *** Interact with federal regulators i n the making of suitable regulations. The basic goal i n trace analysis work i s to be able to f i r s t report the numbers. The second goal i s to indicate the error of each number. The third goal i s for the user of the information, whether i t be an agency, a newspaper reader, or the l i k e , to accept the reports as having error, look for i t , and then use those facts to give information and to form knowledge (27). Acknowledgment This paper i s published as Journal Series Paper No. Pennsylvania A g r i c u l t u r a l Experiment Station.

7609 of

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RECEIVED September 28, 1987

In Detection in Analytical Chemistry; Currie, L.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.